feat: implement Time<S> class template and CivilTime struct; update t…

[VPRamon]

This pull request significantly refactors the time scale system in the tempoch C++ library to introduce a more extensible, trait-based approach for representing and converting between time scales. The main changes include replacing the old hardcoded time types and conversion logic with a generic Time<S> template, introducing scale tags and traits, and updating type aliases and documentation for clarity and extensibility.

The most important changes are:

1. Introduction of Scale Tags and Traits

• Added a new scales.hpp header that defines time scale tag types (JDScale, MJDScale, UTCScale) and the TimeScaleTraits/TimeConvertTraits trait classes, enabling generic and extensible handling of time scales and conversions. This allows for easier addition of new time scales in the future.

2. Refactoring of Type Aliases and Documentation

• Updated the main umbrella header tempoch.hpp to document and re-export the new Time<S> template and its aliases (JulianDate, MJD, CivilTime, UTC), and to include the new scales.hpp and reorganized headers.

3. Simplification of Time Type Definitions

• Replaced the previous concrete classes for JulianDate, MJD, and UTC in time.hpp with type aliases to the generic Time<S> template, and moved all implementation to time_base.hpp and scales.hpp. This reduces code duplication and centralizes logic.

4. Period and Traits Adjustments

• Updated Period<T> and its traits: clarified in period.hpp that TimeTraits<Time<S>> specializations are now provided automatically for all scale-based types, and provided a manual specialization for CivilTime. [1] [2]
• Changed the definition of UTCPeriod to use Period<CivilTime> instead of Period<UTC>, reflecting the new canonical civil time type.

These changes modernize the codebase, improve extensibility for new time scales, and simplify the interface for users and maintainers.