Pygaia

* Documentation enhancements and fixes.

* Fixed error in DR5 estimated end of data collection time.

* Version change to fix automated PyPI publication.

* Notebooks with examples integrated into documentation.

* Added the Gaia data release time spans to the utils module.
* [Error in epoch propagation code fixed](https://github.com/agabrown/PyGaia/pull/14)
* Unit tests updated to work with [pytest](https://docs.pytest.org/en/stable/index.html)

* Renamed master branch to main.
* Added new performance predictions for photometry and radial velocities (appeared on Gaia science performance pages after
Gaia DR3).
* Changed repository layout.
* Updated to using pyproject.toml configuration and using pip to install from source.
* Cleaned up documentation and published on readthedocs.io.
* Changed license.