Chore: extract calculations from interface and clean up

[kzsigmond]

This pull request refactors both the legacy.py and pyci.py FanCI interface modules to streamline the conversion of occupation arrays to Slater determinants. It removes the energy (aka variational-like) objective code, since the classes are for the projected Schrodinger equation. The PR removes unused imports and redundant code.

Refactoring and code simplification:

• Replaced repeated logic for converting occupation arrays to Slater determinants with a single utility function, convert_pyci_occs_to_fanpy_sds, in all relevant methods (compute_overlap, compute_overlap_deriv, and compute_overlap_double_deriv) in both legacy.py and pyci.py. [1] [2] [3] [4] [5] [6]
• Removed the import and usage of the slater module in both files, as its functionality is now encapsulated in the utility function. [1] [2]
• Removed unused imports, specifically cma, minimize, and related optimization methods. Note that these methods do not work with the Projected Schrodinger objective. They will be added to the variational interface class. The supported optimization modes are 'root' and 'lstsq' for the ProjectedSchrodingerFanCI class. [1] [2] [3]

API and logic changes:

• Simplified the compute_objective and compute_jacobian methods in legacy.py by removing the code paths for the 'energy' objective type, focusing exclusively on the 'projected' objective. [1] [2] [3]
• Fixed argument handling in compute_overlap_double_deriv to clarify the logic for string and array inputs. [1] [2]

Documentation and comments:

• Updated class docstrings to clarify the role of pyci.py as the Projected Schrodinger objective class for PyCI.
• Added or updated TODO comments for future improvements and clarifications. [1] [2] [3] [4] [5] [6]