Create a "Priors" class
At the moment users specify a prior in the following way
prior = {'mass_1': 10, 'mass_2': peyote.parameters.Parameter('mass_2', Prior=Unif(0, 100)}
It may be nice to instead have a dedicated data container so that we can do
prior = peyote.empty_prior()
prior.set_fixed('mass_1', 10)
prior.set_prior('mass_2', 'unif', 0, 10)
or something like that (the syntax could be quite different of course).