Pygaia

* Fixed the typo noted in https://github.com/agabrown/PyGaia/pull/4

* Minor fixes to code, documentation link in EpochPropagation class updated.

* This version is for the post-Gaia EDR3 performance predictions which are extrapolated from the actual Gaia (E)DR3
performances. See [https://www.cosmos.esa.int/web/gaia/science-performance](https://www.cosmos.esa.int/web/gaia/science-performance)
* __MANY BREAKING CHANGES!__ For the pre-Gaia EDR3 version of the code see [https://github.com/agabrown/pygaia-v1
](https://github.com/agabrown/pygaia-v1)
* The performance predictions for astrometry now depend only on G and can only be generated for Gaia (E)DR3/DR4/DR5
. Variations as a function of sky position and colour are not included.
* The code was cleaned up, making it (more) conformant with the Python standards. This again causes many breaking
changes due to renaming of modules and functions.

* Mission length scaling in photometric and radial velocity uncertainties.

* Allow for list of spectral types as input to vradErrorSkyAvg.

* Switched from Basemap to Cartopy in sky.py.
* Epoch propagation added, including the propagation of the covariance matrix.
* Covariance matrix construction from Gaia catalogue columns, including "radial proper motion" term.
* Changed convention for longitude numerical values. They are now between 0 and 2PI instead of -PI and
+PI. __THIS IS A BREAKING CHANGE.__