Test: interface submodule

[kzsigmond]

This pull request introduces several improvements and bug fixes to the fanpy.interface.fanci.alias and related PyCI interface code, with a strong focus on input validation, type safety, and enhanced test coverage. The most significant changes include stricter type and value checks in the Alias class, improvements to interface consistency, and the addition of comprehensive unit tests for legacy, pyci and alias functionality.

Input validation and error handling improvements:

• Added explicit type and value checks in the Alias class constructor and __call__ method to ensure probability vectors are numpy arrays, within the valid range, and 1D (with flattening and warning if not). The __call__ method now checks that n is an integer within valid bounds.
• Improved error handling in the fanpy_ham and pyci_ham property setters in fanpy/interface/pyci.py to raise clear TypeErrors when input types are invalid.

Interface and documentation consistency:

• Updated the mask parameter in fanpy/interface/fanci/pyci.py from a list to a np.ndarray of bools, with updated docstrings to clarify its usage for freezing parameters.
• Fixed a bug in test instance creation for PyCI objects by ensuring the mask is correctly sized and of boolean type.

Testing enhancements:

• Added a new test module tests/test_interface_fanci_alias.py with comprehensive unit tests for the Alias class, covering input validation, warnings, and output correctness.
• Refactored and expanded tests in tests/test_interface_fanci_legacy.py and tests/test_interface_fanci_pyci.py
• Added a tests/interface_utils.py module with reusable fake classes for testing, improving test isolation and maintainability.

** Note on test coverage **

• legacy.py does not cover the ProjectedSchrodingerLegacyFanci class individually. This is because this class is equivalent to the pyci FanCI class.
• pyscf_tools.py has a lower test coverage because the localization function is not well documented, and may have a bug in it. Therefore, we are just testing the easiest case for localization.
• legacy.py & pyci.py: we are testing the most common inputs for the objective class. There are no additional masks, norm_det is set to the default, etc. These are features, we do not actively use, so testing them later should suffice. Especially since the test coverage now is significantly higher.