- Moves the hyperPE likelihood to core
- Adds the GaussianLikelihood
- Update docs
- Update examples

- Moves create_default_prior to the core prior file
- Makes fill_prior a class attribute
- Makes the BBH default priors its own file which is read in when
  figuring out the defaults
- Checks the example all still run

Previously I was setting the noise_log_likelihood to nan in a confusing
way. This removes that and also renames all the liklihoods for clarity.

The injectino parameters do not correspond to the recovered parameters
(two different models).

and time marg before moving onto dist and tiem nad phase marg

- Add doc file on CBC PE
- Add introduction text

ratio looks a bit off: need to figuring out what's going on with the
normalization