Enabling registration of REAL volumes using Mutual Information measure.  Presently the code is only working for 16-bit volumes (that was used to show PET->MPRAGE prototype).  Solution is to convert to 16-bit correctly while binning the voxel values for the MI.

This is also strongly related to a proper binning for REAL volumes, so it is addressed at the same time along with the corresponding binning interface (Sturges formula) in the registration dialogs.