Adds MCMCFollowUpSearch

This adds a class to perform follow ups from semi-coherent to
fully-coherent using an MCMC search.

- Also changes the logic of the MCMC search to check for old data only
  in the run method.
- The nsteps argument exists for the MCMCFOllowUP, but is unused.
- Some work on the plot_walkers to allow for overplotting. It may be
  useful to use this for the usual calls as well so that we don't have
  an initial and walkers plot?