Feature/lf additional models

[nikosarcevic]

This PR expands LFKit's luminosity function model library, validation utilities, and test coverage.

New luminosity function models

• Added gamma-family luminosity function models.
• Added Saunders-family luminosity function models.
• Added additional Schechter-family variants.
• Added non-parametric luminosity function models, including tabulated and binned representations and their redshift- and distance-dependent extensions.

Validation improvements

• Added reusable validation utilities for tabulated, binned, and multidimensional luminosity function grids.
• Improved input validation for non-parametric luminosity function models.

Conditional luminosity functions

• Extended conditional luminosity function generation to support newly added luminosity function models.
• Standardized conditional model naming for automatically generated wrappers.

Testing

• Added comprehensive unit tests for:

  • Gamma luminosity function models
  • Saunders luminosity function models
  • Extended Schechter luminosity function models
  • Non-parametric luminosity function models
  • Validation utilities

• Expanded API and registry tests to ensure newly added luminosity function models are discoverable, registered correctly, and evaluable through the public API.

Closes #13