mrtrix: tensor2metric

[kaitj]

Haven't been able to test this yet, but these 2 output parameters potentially look problematic in tensor2metric:

• value: str
• vector: str

These should create as many files as there are values (to a max of 3) in the num parameter (list[int]) as they are associated

[nx10]

This would need some language feature feature in Styx. Probably wont ever fix this unless there is huge demand. Users are able to dynamically grab these via outputs.root.