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.

bilby/core/sampler/__init__.py

@@ -11,6 +11,7 @@ from .cpnest import Cpnest
 from .dynesty import Dynesty
 from .dynamic_dynesty import DynamicDynesty
 from .emcee import Emcee
+from .kombine import Kombine
 from .nestle import Nestle
 from .polychord import PyPolyChord
 from .ptemcee import Ptemcee
@@ -22,9 +23,8 @@ from . import proposal
 
 IMPLEMENTED_SAMPLERS = {
     'cpnest': Cpnest, 'dynamic_dynesty': DynamicDynesty, 'dynesty': Dynesty, 'emcee': Emcee, 'nestle': Nestle,
-    'ptemcee': Ptemcee,'ptmcmcsampler' : PTMCMCSampler,
-    'pymc3': Pymc3, 'pymultinest': Pymultinest, 'pypolychord': PyPolyChord,
-    'fake_sampler': FakeSampler }
+    'ptemcee': Ptemcee,'ptmcmcsampler' : PTMCMCSampler, 'pymc3': Pymc3, 'pymultinest': Pymultinest,
+    'pypolychord': PyPolyChord, 'fake_sampler': FakeSampler, 'kombine': Kombine}
 
 if command_line_args.sampler_help:
     sampler = command_line_args.sampler_help