Allow for the Python sampler to take more samples after completion.

[m.lykkegaard]

This is more of a feature request. It would be great if, when the MCMC sampler has finished, one could pass something like mcmc.Run(additional_samples) to let the sampler pick up where it left. I often run a bunch of samples and then realise that it hasn’t converged. This is particularly useful with adaptive samplers that get better as they sample.