mrconvert: Reduce template count
mrconvert: Reduce template count

Only use (u)int32, (u)int64 and (c)double for conversion; reduces compile time.
Relates to #3 and #200.

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