[enhancement] Intreface for mrtrix3's transformconvert and mrtransform

[GalKepler]

Summary

I think it'd be helpful to have interfaces for mrtrix3's transformconvert and mrtransform functions. I know mrtransform used to have an interface but it seems to be a bit faulty. The main advantage I see in using mrtrix3's built-in functions rather than other tools' is that when performing such transformations on dMRI data, it's important to apply the same transformations on the gradients' bvec and bval files. Mrtrix3's have an internal mechanism that does exactly that, as mentioned here.