fix two bugs in mrconvert: handle 'end' keyword in -coord specification, and output correct image size when using -coord option
fix two bugs in mrconvert: handle 'end' keyword in -coord specification, and output correct image size when using -coord option

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