Adding sampler

This branch adds a basic sampler. It is not yet complete and there are a few things that need to be ironed out, but it produces posteriors. Running the tutorial/Sample.py file will produce this trace (x1 is the amplitude and x2 is the frequency):

Note: this currentlty requires dynesty to be installed which is not yet a requirement.

Tasks that need to be done

• Update the likelihood to @colm.talbot's likelihood (Done, but leaving both likelihood for future discussion)
• Agree on the input method (currently a dictionary with either Parameter (indicating the parameter should be sampled) or a float (indicating it is fixed)
• Add checking that the input parameters are valid
• Add default priors Leaving this til a later date
• Add summary plots to the standard output? Leaving this til a later date
• Update tutorial to BBH

Thoughts/suggestions welcome

@all