Combine multiple runs

We need a method for combining the output of multiple runs, this will need to be treated a little differently for different samplers.

Hopefully, outputs from nested samplers can all be combined in this way, if we wish. Similar for MCMC. I'm not sure how best, if at all to combine MCMC runs with nest runs.

Related is thinning for auto-correlation length for MCMC samplers, which may or may not be done currently.

Ideally, this will be possible from the results files.