Vary the cosmology used

At the moment, all the cosmological conversions in bilby/gw/conversion.py default to using the astropy.cosmology.Planck15 cosmological parameters. There should be an option to choose a different cosmology, either from the ones avaliable through Astropy or by defining your own (I'd suggest using the infrastructure from Astropy's FlatLambdaCDM, or a more general Cosmology classes avaliable)