General fix up of priors

Things this MR will include

• Make a PriorSet class which is a dictionary containing the priors
• Add a method to read in a PriorSet from file
• Move the BBH default priors to a text file which is read in to a super PriorSet of default priors
• Allow users to choose there default priors file (or a default will be chosen, maybe even based on the source model?)
• Ensure that gw and core don't overlap anymore

@colm.talbot @paul-lasky @moritz.huebner feel free to chip in (and anyone else of course).