allow other distance parameters to be marginalised over

This MR will allow people to specify a Cosmological prior in redshift or comoving_distance and still use distance marginalisation.

There are cases when this will at the least make the limits look neater, e.g., specifying z <= 1 rather than whatever luminosity distance that corresponds to.

I also fixed a lingering bug in the converting between luminosity and comoving distance priors.