support variable transformations within automatic look-up of lnpdraw fields

Support the calculation of lnpdraw for a few simple variable transformations such as

• asymmetric_mass_ratio <--> mass2_source | mass1_source
• cartesian spin components <--> polar spin components
• else?

We may do this by retaining a list of which variates do not have a corresponding logprob_field (instead of immediately raising an exception) and then checking to see whether any of those variates match a set of known transforms for which lnprob_fields do exist. We will need to check whether the new fields conflict with any of the other existing fields as well as actually compute and return the jacobian, etc.

Note, this will almost certainly require changes to gw-distributions as well.