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
Add baryonic matter density paramter and its evolution to cosmology classes
add dark matter only density parameter and its evolution
update doctests for Omega_b
Add baryonic matter density paramter and its evolution to cosmology classes
add dark matter only density parameter and its evolution
do not raise an exception if Ob0 is accessed and None, return None
Mention Omega_b
Consistent treatmemt of `Ob` and `Odm`

Do not pretend that Ob0 is 0.0 if `Odm0` and `Odm` are accessed and `Odm0` is `None`.
test consistent dark matter, baryonic matter, all non-rel. matter evolution
document Omega_baryon attribute/methods and behavior
PEP8 fixes