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?

examples/injection_examples/basic_tutorial.py

@@ -41,7 +41,7 @@ for key in ['a_1', 'a_2', 'tilt_1', 'tilt_2', 'phi_12', 'phi_jl', 'phase', 'psi'
 priors['luminosity_distance'] = tupak.prior.create_default_prior(name='luminosity_distance')
 
 # Initialise Likelihood
-likelihood = tupak.likelihood.Likelihood(interferometers=IFOs, waveform_generator=waveform_generator)
+likelihood = tupak.likelihood.Gaussian(interferometers=IFOs, waveform_generator=waveform_generator)
 
 # Run sampler
 result = tupak.sampler.run_sampler(likelihood=likelihood, priors=priors, sampler='dynesty', npoints=100, walks=10,