This allows the waveform generator to be given None for the sampling
frequency and duration (in this state, it therefore can't be used to
generate an injection). Then, when passed to the likelihood, a check if
performed to see if those attributes are set. If they are None, they are
set from the data, and if they disagree with the data, an error is raised.

- time_duration -> duration
- fix spelling error tiime -> time
- frequencies -> frequency_array

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