Draft: BUG: Make sure conversion_function is propagated to the priors

Some applications (e.g. tests of GR) need to pass custom conversion functions to the prior class while defining a prior. Currently, bilby_pipe sets the conversion function to the default_conversion_function as defined in the prior class. However, I feel that the correct behaviour would be to use the conversion_function passed in the config file to define the priors. This MR proposes a change to this effect.

PS: This was noticed by @purnima.narayan @nathan-johnson-mcdaniel while performing some FTI runs with bilby_tgr.