Adding the Kombine Sampler

This modifies/uses the emcee EnsembleSampler wrapper to be used as a wrapper to allow sampling using Ben Farr's kombine code (It has a mostly similar interface as emcee so it was not a huge stretch to extend for kombine to be used).

https://github.com/bfarr/kombine

I am not sure if this will be useful to have kombine has an available sampler but I know Ben (my advisor) has been interested to see it work in bilby.