Inconsistent use of `priors` and `prior`

In run_sampler, the argument to give the prior is priors=, while in bilby.gw.likelihood.GravitationalWaveTransient it is prior. To be consistent across the project we should pick one or the other

It is an easy change, but I'm opening an issue to allow people to voice an opinion on whether it should be prior or priors. My vote is priors.