Supporting Python 3.8 - 3.10

Small fixes to package including documentation, CI, and updating the supported python versions to 3.8 through 3.10.

Update default GSC to 2.4.2.1

Update default Guide Star Catalog to 2.4.2.1, which fixes inaccurate magnitudes for some guide stars.

Improve Algorithms to match GSS

Update the algorithms used in the tool to better match those in theGuide Star Selection System; includes adding an R-Z calculation option and breaking apart the SDSS and GSC2 calculations, among other changes.

Update default GSC to 2.4.2

Update default Guide Star Catalog to 2.4.2, remove support for Python 3.5 and add Python 3.8, and add direct FGS magnitude to count rate and count rate to FGS magnitude functions.

Initial Release

This release includes the querying of GSC2.4.1, which contains TMASS, SDSS, and GSC stars.