mrtrix3.algorithm: Fix for branch & version switch
mrtrix3.algorithm: Fix for branch & version switch

Prevents a deep Python exception in the specific case where one executes an algorithm-based script using Python 2, switches to a code branch where a particular algorithm is no longer present, and then tries to run the same script using Python 3.

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