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 dark matter only density parameter and its evolution
update doctests for Omega_b
add dark matter only density parameter and its evolution
update doctests for Omega_b
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