This MR adds a new optional hyperpolic background form to the RICH Cherenkov fitter. The default remains the polynomial form.
This MR has no bearing on the HLT or Brunel, this fitter is not used there. I include it for 2017-patches as I would like to test the new fitter at the pit, where it is used during the refractive index calibration and our monitoring of the resolutions. hence it would be very useful to have it in the 2017-patches release.