mrmetric: added meansquared metric from registration. changed default interpolator to linear. tidied up some duplicate code mess.
mrmetric: added meansquared metric from registration. changed default interpolator to linear. tidied up some duplicate code mess.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3