add jacobian to prior

This adds the option to pass a Jacobian function to the PriorDict. This allows non-trivial priors to be placed on non-sampled parameters.

This is a method to reproduce the uniform in component mass priors while sampling in chirp mass and mass ratio.

I'm not sure whether we want this feature as it complicates the interaction with some nested samplers, thoughts?

If we decide we do want it I'll make sure to add an example of using this somewhere.