101 complete basis set migration (#102)

- Migrate / Add the following basis sets:

  - plane waves (no pseudopotentials)
  - LAPW
  -- revised structure centered on l-channels (similar to Gulans' uploads) 
  -- automatic type detection (new)
  -- scaffhold generator (new)
  - GPW
  - AtomCentered (just placeholder)

- Add testing for all features (new)
- Touch up `Mesh`
- Add normalization flow decorators to `general.py`

---------

Co-authored-by: ndaelman <[email protected]>
Co-authored-by: nathan <[email protected]>

docs/model_method/basis_sets.md

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+# Basis Sets
+
+The following lays down the schema annotation for several families of basis sets.
+We start off genercially before running over specific examples.
+The aim is not to introduce the full theory behind every basis set, but just enough to understand its main concepts and how they relate.
+
+## General Structure
+
+Basis sets are used by codes to represent various kinds of electronic structures, e.g. wavefunctions, densities, exchange densities, etc.
+Each electronic structure is therefore described by an individual `BasisSetContainer` in the schema.
+
+Basis sets may be partitioned by various regions, spanning either physical / reciprocal space, energy (i.e. core vs. valence), or potential / Hamiltonian.
+We will cover the partitions per example below.
+Each `BasisSetContainer` is then constructed out of several `basis_set_components` matching a single region.
+Sometimes, a region is defined in terms of another schema section, e.g. an atom center (`species_scope`) or Hamiltonian terms (`hamiltonian_scope`).
+
+Note that typically, different kinds of regions also have different mathematical formulations.
+Each formulation has its own dedicated section, to facilitate their reuse.
+These are all derived from the abstract section `BasisSetComponent`, so that `basis_set_components: list[BasisSetComponent]`.
+
+Generically, `BasisSetComponent` will allude to the the formula at large and just focus on capturing the _subtype_, as well as relevant _parameters_.
+The most relevant ones are those that most commonly listed in the Method section of an article.
+These typically also influence the _precision_ most.
+Extra, code-specific subtypes and parameters can be added by their respective parsers.
+
+This then coalesces into the following diagram:
+
+```
+ModelMethod
+└── NumericalSettings[0]
+└── ...
+└── NumericalSettings[n] = BasisSetContainer
+                            └── BasisSetComponent[1]
+                            └── ...
+                            └── BasisSetComponent[n]
+                                └──> AtomsState
+                                └──> BaseModelMethod
+```
+
+## Plane-waves
+
+Plane-wave basis sets start from the description of a free electron and use Fourier to construct the representations of bound electrons.
+In reciprocal space, the basis set can thus be thought of as vectors in a Cartesian* grid enclosed within a sphere.
+
+The main parameter is the spherical radius, i.e. the _cutoff_, which corresponds to the highest frequency representable Fourier frequency.
+By convention, the radius is typically expressed in terms of the kinetic energy for the matching free-electron wave.
+`PlaneWaveBasisSet` allows storing either `cutoff_radius` and `cutoff_energy`.
+It can even derive the former from the latter via normalization.
+
+### Pseudopotentials
+
+Under construction...
+
+## LAPW
+
+The family of linearized augmented plane-waves is one of the best examples of region partitioning:
+
+- first it partitions the physical space into regions surrounding the atomic nuclei, i.e. the _muffin-tin spheres_, and the rest, i.e. the _interstitial region_.
+- it then further partitions the muffin tins by energy, i.e. core versus valence.
+Note that unlike with pseudpotentials, the electrons are not abstracted away here.
+They are instead explicitly accounted for and relaxed, just via a different representation.
+Hence, LAPW is a _full-electron approach_.
+
+The interstitial region, covering mostly loose bonding, is described by plane-waves (`APWPlaneWaveBasisSet`). [1]
+The valence electrons in the muffin tin (`MuffinTinRegion`), meanwhile, are represented by the spherically symmetric Schrödigner equation. [1]
+They follow the additional constraint of having to match the plane-wave description.
+In that sense, where the plane-wave description becomes too expensive, it is "augmented" by the muffin-tin description.
+This results in a lower plane-wave cutoff.
+
+The spherically symmetric Schrödigner equation decomposes into an angular and radial part.
+In traditional APW (not supported in NOMAD), the angular and radial part are coupled in a non-linear fashion via the radial energy (at the boundary).
+All versions of LAPW simplify the coupling by parametrizing this radial energy. [1]
+
+The representation vector is then developed in terms of the angular basis vectors, i.e. $l$-channels, each with their corresponding radial energy parameter.
+This approach is -confusingly- also called _APW_.
+It is typically not found standalone, though.
+Instead, the linearization introduces a secondary representation via the first-order derivative of the basis vector (function).
+Both vectors are typically developed together.
+This technique is called linearized APW (LAPW). [1]
+
+Other formulas have been experimented with too.
+For example, the use of even higher-order derivatives, i.e. superlinearized APW (SLAPW). [2, 3]
+All of these types are captured by `APWOrbital`, where `type` distinguishes between APW, LAPW, or SLAPW.
+The `name` quantity
+
+Another option is to stay with APW (or LAPW) and add standalone vectors targeting specific atomic states, e.g. high-energy core states, valence states, etc.
+These are called _local orbitals_ (lo) and bear other constraints.
+Some authors distinguish different vector sums with different kinds of local orbitals, e.g. lo, LO, high-dimensional LO (HDLO). [2, 4]
+Since there is no community-wide consensus on the use of these abbreviations, we only utilize `lo` via `APWLocalOrbital`.
+
+In summary, a generic LAPW basis set can thus be summarized as follows:
+
+```
+LAPW+lo
+├── 1 x plane-wave basis set
+└── n x muffin-tin regions
+    └── l_max x l-channels
+        ├── orbitals
+        └── local orbitals ?
+```
+
+or in terms of the schema:
+
+```
+BasisSetContainer(name: LAPW+lo)
+├── APWPlaneWaveBasisSet
+├── MuffinTinRegion(atoms_state: atom A)
+├── ...
+└── MuffinTinRegion(atoms_state: atom N)
+    ├── channel 0
+    ├── ...
+    └── channel l_max
+        ├── APWOrbital(type: lapw)
+        └── APWLocalOrbital ?
+```
+
+[1]: D. J. Singh and L. Nordström, \"INTRODUCTION TO THE LAPW METHOD,\" in Planewaves, pseudopotentials, and the LAPW method, 2nd ed. New York, NY: Springer, 2006.
+
+[2]: A. Gulans, S. Kontur, et al., exciting: a full-potential all-electron package implementing density-functional theory and many-body perturbation theory, _J. Phys.: Condens. Matter_ **26** (363202), 2014. DOI: 10.1088/0953-8984/26/36/363202
+
+[3]: J. VandeVondele, M. Krack, et al., WIEN2k: An APW+lo program for calculating the properties of solids, _J. Chem. Phys._ **152**(074101), 2020. DOI: 10.1063/1.5143061
+
+[4]: D. Singh and H. Krakauer, H-point phonon in molybdenum: Superlinearized augmented-plane-wave calculations, _Phys. Rev. B_ **43**(1441), 1991. DOI: 10.1103/PhysRevB.43.1441
+
+## Gaussian-Planewaves (GPW)
+
+The CP2K code introduces an algorithm called QuickStep that partitions by Hamiltonian, describing
+
+- the kinetic and Coulombic electron-nuclei interaction terms of a Gaussian-type orbital (GTO).
+- the electronic Hartree energy via plane-waves.
+
+This GPW choice is to increase performance. [1]
+In the schema, we would write:
+
+```
+BasisSetContainer(name: GPW)
+├── PlaneWaveBasisSet(hamiltonian_scope: [`/path/to/kinetic_term/hamiltonian`, `/path/to/e-n_term/hamiltonian`])
+└── AtomCenteredBasisSet(name: GTO, hamiltonian_scope: [`/path/to/hartree_term/hamiltonian`])
+```
+
+For further details on the schema, see the CP2K parser documentation.
+
+[1]: J. VandeVondele, M. Krack, et al., Quickstep: Fast and accurate density functional calculations using a mixed Gaussian and plane waves approach,
+_Comp. Phys. Commun._ **167**(2), 103-128, 2005. DOI: 10.1016/j.cpc.2004.12.014.