dwi2response: New algorithm 'manual'
dwi2response: New algorithm 'manual'

This duplicates the functionality of the msdwi2response script, but also has the following:
- Can be used in a single-shell context using the -shells option.
- Fibre directions can be pre-defined using the -dirs option (a tensor calculation will take place if this is not provided) (this functionality has also been duplicated in the 'msmt_5tt' algorithm

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