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
move _float_or_none from FLRW method to helper function
fix typo
clarify docstring of Odm