Generalize ProjTTNSum, replace ProjTTNApply with ProjOuterProdTTN

[b-kloss]

This PR intends to generalize the design of ProjTTNSum to be Vectors of an arbitrary concrete subtype of AbstractProjTTN.
Changes:

• Made ProjTTNSum subtype of AbstractProjTTN and and represent a sum arbitrary AbstractProjTTN
• Removed type restrictions on position, in line with recent changes to position for AbstractProjTTN.
• Renamed ProjTTNApply to ProjOuterProdTTN and redefine functionality to be in line with that of ProjTTN for an operator O of the form O=A|v><v|A^dagger. The prior functionality is accessible via contract_ket, which is used e.g. in contract_updater, the backend for variational compression of |x> = \sum_i A_i |b_i>.

ToDo:

• implement tests for sum of ProjTTNApply
• generalize contract solver
• discuss whether the parametrization is sensible
• discuss problematic constructors
• define field accessor functions instead of accessing fields directly
• remove outdated comments

[codecov-commenter]

Codecov Report

Attention: 22 lines in your changes are missing coverage. Please review.

Comparison is base (cf59738) 72.87% compared to head (1ce884d) 73.24%.

Files	Patch %	Lines
src/treetensornetworks/projttns/projttnsum.jl	65.62%	11 Missing ⚠️
src/treetensornetworks/solvers/contract.jl	78.37%	8 Missing ⚠️
src/treetensornetworks/projttns/abstractprojttn.jl	66.66%	2 Missing ⚠️
...rc/treetensornetworks/projttns/projouterprodttn.jl	98.33%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #132      +/-   ##
==========================================
+ Coverage   72.87%   73.24%   +0.36%     
==========================================
  Files          68       68              
  Lines        3982     4033      +51     
==========================================
+ Hits         2902     2954      +52     
+ Misses       1080     1079       -1     

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[mtfishman]

@b-kloss this looks like a promising approach to supporting more general sums of projected operators. Definitely preferable to defining a new ProjTTNApplySum type, if it seems easy enough to generalize the current ProjTTNSum type.

[b-kloss]

If tests pass this is ready for review from my side.

[mtfishman]

Looks good, thanks! Over all a nice improvement which extends the functionality a lot (but also cleans up the code).