Add a zero-likelihood mode

Adds a test-mode for running with zero likelihood.

• Adds a ZeroLikelihood which can be instantiated from any other likelihood as a drop-in replacement
• To use on any example, run with --bilby-zero-likelihood-mode in the command line
• A test has been added which runs a 15d CBC problem and checks the 1D posteriors agree with the priors.
• The next is added to the C.I nightly test