Test: increase geminal test coverage

[kzsigmond]

This pull request significantly expands and improves the test coverage for coupled cluster (CC) wavefunction classes, especially around excitation operator assignment and overlap calculations. It introduces new tests for generalized and spin-adapted AP1roGSD classes, enhances error handling and validation for excitation operator assignment, and adds comprehensive tests for overlap and its derivatives (including edge cases and parameterizations).

The most important changes are:

New and Expanded Test Suites:

• Added new test files for AP1roGSDGeneralized and AP1roGSDSpin, covering assignment of excitation ranks, validation of allowed excitation operators, and verifying the generation of excited determinants. [1] [2]
• Enhanced the test suite for PCCD and related AP1roGSD classes with tests for overlap calculations, edge cases, and derivatives (including finite difference checks and symmetry properties). [1] [2] [3] [4]

Validation and Error Handling for Excitation Operator Assignment:

• Added and improved tests for assign_exops in APset1roGD and APset1GroSD, including validation of input types, value ranges, set disjointness, and occupied orbital checks. [1] [2] [3]

Improvements to Excitation Operator Assignment in Implementation:

• Modified PCCD and its child classes to pass indices directly to the base class, ensuring correct handling of excitation operator assignment and reducing redundant calls.

Test Refactoring and Organization:

• Refactored and organized test files to group related tests (e.g., assignment, overlap, derivatives), improving readability and maintainability. [1] [2]

Minor Additions and Imports:

• Added missing imports and utility functions to test files to support new tests (e.g., ground, vir_indices). [1] [2]