Remote/parallel model computation without ipyparallel, bestfit_emcee

[JelleAalbers]

This adds:

• An n_cores option to the prepare method of likelihood functions, to allow parallel model computation on several machines.
• Experimental (as in, I have no idea if it works yet): A source can now make a file in pdf_tasks which contains all information needed to compute the pdf. The bluethaw script can turn this into a pdf_cache file. Using this you can compute models remotely / on a batch queue.
• Adds bestfit_emcee, to fit likelihoods using emcee (and optionally get pre-made corner plots etc).

The old ipyparallel paralellization interface is preserved for compatibility.