Making slightly different cosmologies is awkward

Currently it's a bit cumbersome to make a version of a cosmology you already have but with small things changed.  It would be nice if we supported making a copy of a cosmological object but with a specified set of values changed.

A common use case, for example, might be to step through changes in Omega matter but keep all other values the same (flatness, H0, etc.).  Right now the user has to remember all the other values they passed to the constructor to change any of the cosmological variables.