feat: add FFI-backed time scales and conversion traits for various ti…

[VPRamon]

This pull request significantly expands support for time scales in the tempoch library, introducing new types for a variety of astronomical and atomic time standards. It implements generic infrastructure for time scale traits, conversion, and arithmetic, and adds user-friendly type aliases for these new scales. Additionally, it introduces a UT1-specific method for computing ΔT (difference between Terrestrial Time and Universal Time).

The most important changes are:

New time scale types and aliases

• Added new scale structs for TT, TAI, TDB, TCG, TCB, GPS, UT1, JDE, and Unix time in scales.hpp, each with documentation and intended storage format.
• Introduced convenient type aliases in time.hpp for each new time scale, e.g., using TT = Time<TTScale>, using TAI = Time<TAIScale>, etc., making these types easier to use throughout the codebase.

Time scale traits and conversion infrastructure

• Implemented a generic JDBackedScaleTraits template in scales.hpp to handle arithmetic and civil time conversion for all JD-backed scales, reducing code duplication and standardizing behavior.
• Added explicit specializations of TimeScaleTraits for each new scale, using the generic JD-backed helper, and provided human-readable labels for each.
• Expanded TimeConvertTraits to support direct and indirect (via JD) conversions between all supported time scales, including identity conversions and explicit specializations for JD↔{TT, TAI, TDB, TCG, TCB, GPS, UT1, JDE, UnixTime}.

UT1-specific functionality

• Added a delta_t() method to Time<UTScale> (Universal Time), which computes ΔT (TT - UT1) in seconds, enabled only for UT1 time points.

Submodule update

• Updated the tempoch submodule to the latest commit.