Custom BGMN params

[Andrea-gm]

Summary

Major changes:

• feature 1: Added the custom_lines argument to the cif2str function to support injecting arbitrary BGMN string parameters, mathematical equations, and defect models directly into the generated .str files.
• feature 2: Added the element_params_map argument (typed as dict[str, dict]) to dynamically override or add parameters to specific Wyckoff lines. By using Python's .update() method, this cleanly removes the previous reliance on the hardcoded TDS string, allowing users to explicitly define independent parameters like {"TDS": "BO", "Occ": "OccO"} per element. Includes wildcard (*) support to easily apply global variables across the entire crystal structure.
• fix 1: Resolved the limitation where high-temperature and highly disordered refinements (e.g., alpha-phase K2CO3) would crash or fail to converge due to the rigid, hardcoded room-temperature TDS=0.010000 default.

Todos

If this is work in progress, what else needs to be done?

• fix 2: Verify that cif2str kwargs are properly exposed and passed through the higher-level RefinementPhase and do_refinement manager classes so users can access these new arguments seamlessly from the top-level API.

Checklist

• All existing tests pass.
• Tests have been added for any new features/fixes.
• Docstrings have been added in the Google docstring format.