Unit tests for distributions

Merged Alison Hale requested to merge unit-tests-for-distributions into master

Unit tests and examples for DiscreteTimeStateTransitionModel Class now with a log_prob test to check maxima. Update to log_prob() in this class see Issue 12 however note there's a possibility this could have side effects on Chris's covid code. If further unit tests are required in gemlib then new issues and corresponding branches should be raised.

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