This reverts commit 2ab4bddd.

For some reason, the list comprehension method of setting the IFOs
intereacted badly with the example_test method. This doesn't change
anything, but just makes it a straightforward for loop.

Also removes the failing detector tests.

- Adds in comments from Denyz, and general tidy up.
- Closes #83

Rather than passing around a list of interferometers, have a defined
set. Motivation: in several places we had dangerous calls to
interferometers[0].attribute which will get confusing if not all IFO's
have the same duration etc. This enforces that they do.

- 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).