Use the system monospace font in the docs
Allow for custom datatype names in votable

This should allow end users to workaround VOTable files that contain
invalid datatypes.

Also adds two datatypes seen in Vizier tables: "unsignedInt" and "unsignedShort".
Update docs about datatype mappings.
Add changes entry.
Implementation of wcs_to_celestial_frame
Minor improvements
Make wcs_to_celestial_frame extensible
Added tests and docstring for ``wcs_to_celestial_frame``
Skip doctest
Added changelog entry
Fix parallel testing of test_earth.py
Deprecate (and fix) CartesianPoints
Add changelog entry
Move chagelog entry for #2727 to v0.4.1
Add development/affiliated-packages.rst docs page.

At the moment this is a copy of the `package-template/README.rst` page,
but the idea is that this documentation is restructured after moving it here.
Fix formatting of unusual unit bases.
Merge pull request #2727 from eteq/deprecate-cartesianpoints

Deprecate (and fix) CartesianPoints
Merge pull request #2798 from mdboom/unit/obscure-roundtrip

Originally valid FITS unit cannot be formatted
Merge pull request #1601 from mdboom/docs/make-monospace-font-match

Fix monospaced fonts in docs
Add a six submodule that reloads bundled or system six