WIP: uniform in comoving volume prior; sampling in redshift and source masses.

Implementing (redshifted) uniform in comoving volume distance prior, and sampling in redshift and source frame chirp mass a la https://bugs.ligo.org/redmine/attachments/4962/0001-cosmo-patch.patch

All credit and thanks go to @salvatore-vitale who wrote this code in 2016. This is an attempt to apply Salvo's patch to master, with @vivien's help.