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
Pass open() exceptions in fits.open.
Fix typo in scheme comparison; update tests.
Add changelog entry for #2756/#2785.
Use a registry of known formatter classes.  This has the potential advantage of enabling user-defined formatters.  astropy.units.format.__init__ should still import all the built-in formatters to ensure they are added to the registry, so this is a minimal change.
Allow unit format names to be case-insensitive (normalizing to lowercase) and include a type check in case the format is not a valid string.
Ensure no Python modules under astropy/utils/compat are excluded from the source distribution