Rename likelihood

How do people feel about changing the name of the main likelihood class? I have a couple of reasons why.

1. We currently call the kind of awkward likelihood.Likelihood class.
2. It isn't very descriptive, I'm not sure what an optimal name is but MatchedFilter or Gaussian would be more descriptive.
3. If we want to extend to different use cases, even within GW astrophysics, we'll need more likelihoods. For example, population analysis needs a radically different likelihood. Also, if you're doing a cross-correlation search you may want a different likelihood.
4. I feel like there's an opportunity for TUPAK themed likelihoods (although I won't be generating them as I know nothing about rap).

I understand this would change the UI so I think this should be a high priority if we decide to rename it.

@paul-lasky @gregory.ashton @rory-smith @eric.thrane @moritz.huebner