New time scales and conversions

[VPRamon]

This pull request adds support for the IAU-defined coordinate time scales TCG (Geocentric Coordinate Time) and TCB (Barycentric Coordinate Time), including their conversion logic and documentation. It also improves the accuracy and clarity of the TT↔TDB conversion and updates documentation and tests accordingly.

New time scales and conversions:

• Added TCG (Geocentric Coordinate Time) and TCB (Barycentric Coordinate Time) types, including their mathematical definitions, documentation, and conversion logic to and from TT, following IAU 2000/2006 resolutions.
• Updated macro and exports to include TCG and TCB in time scale conversions and public API. [1] [2]

Improved TT↔TDB conversion:

• Enhanced the tt_to_tdb function to use the Fairhead & Bretagnon (1990) expression, matching USNO Circular 179, for improved accuracy and documentation.

Documentation updates:

• Updated module-level and type-level documentation to include TCG and TCB among the supported time scales. [1] [2]

Testing:

• Added comprehensive tests for TCG and TCB conversions, including offset checks at J2000 and round-trip accuracy.

[github-actions]

Package	Line Rate	Complexity	Health
src	91%	0	✔
Summary	91% (846 / 933)	0	✔