Inter-modal registration: Use multi-contrast

[Lestropie]

Currently, the code does the following:

1. Generate pseudo-T1w image from b=0
2. Generate pseudo-b=0 from T1w
3. Register between DWI-pseudo-T1w and T1w
4. Register between T1w-pseudo-b=0 and DWI b=0
5. Average the results of 3 and 4

It only now occurs to me that this would probably be better handled using multi-contrast registration. Steps 3--5 could be done in a single mrregister call, by providing two volumes within each of the two inputs.