Add multivariate gaussian to core likelihood

This likelihood currently lives in the respective example script. It should be in the core likelihoods for easier testing. @matthew-pitkin

changed milestone to %0.6.5

added Example/Tutorial Refactoring labels

created merge request !716 (merged) to address this issue

mentioned in merge request !716 (merged)

closed via merge request !716 (merged)

mentioned in commit dc202662

unassigned @moritz.huebner