Draft: Add option for callable proposal_cycle to generate proposal_cycle dynamically

I found in the course of performing analyses with bilby_mcmc that I would like to make my own proposal cycles, without necessarily hardcoding them as explicitly identified options within get_proposal_cycle. This does that by instead adding a check for if the passed proposal_cycle parameter is a callable, in which case it assumes that that function has a signature like that of get_proposal_cycle (hence allowing for inclusion of the specific priors), but lacking the string designator.

I'm not sure this is the best way around this, but I did find it useful for my own purposes.