[FIX] MRTrix3 change inputs position for MRTransform

[manuegrx]

If several arguments are used (for example invert and linear_transform) they are not all take into account (and so it is not the same behavior as the native mrtrix3 command). I think that it is because they are all set to position = 1 I think it is not necessary to precise position for those arguments as argstr is already filled with an information about position .