Cosmology
This MR allows sampling in redshift and comoving distance. It uses Astropy
functions methods and calculating redshift is pretty slow, but it's functional and sampling in redshift is fast enough for PE.
This is functional so I'd like to get this in soon, we can make the calculations faster without changing anything for the user.
We are restricted to a hard-coded cosmology (Planck 15), we should generalise this in future.