Updating priors

Make core.prior completely independent of all gw.prior, continuing from https://git.ligo.org/Monash/tupak/merge_requests/56.

• Remove the GW specific default latex labels from core, this method can be overwritten in gw.
• Move the binary_black_holes.prior from core to gw?
• Have an empty PriorSet.test_redundancy which just does nothing which can be overwritten by whichever prior set you like, this will naturally be done by defining a subclass of PriorSet in gw/prior.py.
• Make create_default_prior require the second argument.
• Move everything currently in core/prior_files to gw/prior_files (or something similar). We'd also need to fix the setup.py file to see this, but its all gw-specific.