Generalise convert to flat in component mass

The function to convert to a flat in component mass prior in post-processing, https://git.ligo.org/lscsoft/bilby/blob/master/bilby/gw/prior.py#L29, should be generalisable to accept non-uniform priors in chirp mass/ratio as the starting point.

This may have some overlap with the prior jacobians in !476 (closed).