Add baryonic matter density paramter and its evolution to cosmology clas...

...ses

As discussed in #2707, this PR adds the baryonic density to all cosmology classes. It is a new keyword argument at class instantiation. If a cosmology does not have a value for Omega_b it is left at `None` and trying to access it raises a `ValueError`.
I don't think `_float_or_none` needs to be attached to one of the Cosmolgy classes -- it might be better off as a function in `cosmology.core`.