Add hardcoded determinant ratio wavefunction

1. Parameters are assumed to be given as a flattened array of matrices
   of shape(nelec, nspin)
2. Denominators are differentiated from numerators by with attribute
   `numerator_mask`
3. Methods, `get_columns`, `get_matrix`, and `decompose_index`, have been added
   to navigate the different matrices from the flattened parametered.
   Should a different parameter structure be used (e.g. each matrix
   having different shape from (nelec, nspin)), then only these methods
   need to be changed.
4. `_olp` and `_olp_deriv` have been separated out of the load_cache in
   anticipation of usability improvements (Issue #62)

Issues:
1. `load_cache` will need to move to the base parent class
2. `assign_params` is redundant (and can move to the parent class)
3. `np.logical_not` was used instead of the unary operator `~` or `-`

wfns/wfn/quasiparticle/__init__.py

@@ -0,0 +1 @@
+"""Module for creating wavefunctions related to quasiparticles."""