1) Rename all tests to the pytest standard naming convention
2) Update to use pytest to run the tests and collect coverage
3) Simplify gitlab CI script using above changes

Partial solution to #181

1) If sigma is not given, then it will not be sampled over (or a warning
raised that you didn't specify a prior for sigma).
2) Comments added to explain the logic throughout
3) Tests added for the Gaussian likelihood

This updates to an anaconda python image and removes redundant tests
rerunning examples so that the tests finish in a more reasonable amount
of time. Many requirements are also moves to an
optional_requirements.txt file.

A non-ideal fix to get the coverage badge to work. Seemingly it was
failing before because it now sees tupak as being in the dist-packages.
I've added a hard path to that directory so that at least the numbers
are correct.

- Force version
- Print version information
- Simplify source directory calling

- Moves setup to a before_script
- Remove lalsuite from requirements for CI to prevent it from failing