Expose ability to fit/fix various parameters in MCMCfitter

[Mikea1985]

We chatted about this when you visited. You were right that the ability to fit only certain parameters was sort of already there, although not as separate functions that the user can call directly. I have added a little bit, so that the user can turn on or off the fit of XY with M fixed, fit of M with XY fixed and the fit of XYM together.