Add a Gaussian prior mass option

For the model comparison paper we want to use a Gaussian mass prior.

It turns out this isn't an option in lalinference, but most of the machinery to do this is present. We just need to hook some things up. This is tested here:

https://galahad.aei.mpg.de/~spxiwh/LVC/aLIGO/O2/PE_runs/lalinf_nest/eos_bns_runs/tests/sly_test2/lalinferencemcmc/TaylorF2threePointFivePN/1187008882.45-0/V1H1L1/posplots.html

with a zero-loglikelihood, and we've already started production runs using this patch for the model comparison paper.