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?