FPGATrackSim: implement flag for changing chi2ndof cut in overlap removal

This is something we had in HTTSim but may have gotten lost when the configuration was refactored. I added a "chi2cut" flag that sets the MinChi2 property on the overlap removal tool, so (at least for the linear fitter!) it is now possible to change this.

Also as part of this, I added a histogram for the chi2ndof of tracks passing overlap removal to the monitoring tool, as that is how we were deriving chi2 cuts in the simulation studies done with HTTSim.

This should be very straightforward and not cause any problems with the currently open merge requests (at least, I hope). Tagging @jahreda and @tbold.