Rename likelihood

This MR changes the name of the default likelihood from Likelihood to Gaussian, see https://git.ligo.org/Monash/tupak/issues/55.

Currently, the old version still exists in the file. If a user calls likelihood.Likelihood is will still work but give them a warning saying that the likelihood is deprecated. Do we want this? Or should it just fail and remove it now to avoid confusion later?

Is this good to merge @paul-lasky ?

Closing as this was done (with a different name) in 911ebc9c

closed