Wrap astropy z_at_value for v5.

Closes bilby_pipe#231 (closed).

The most recent release of astropy (v5, py38+) introduced a slew of changes to the cosmology module including a dimensionless unit for redshift.

This has broken our conversion.

The MR wraps the z_at_value function to always return a float (the astropy<5 behaviour). It's a little ugly but does what we need.