dwimotioncorrect: Convenient output of amp-transformed data

[Lestropie]

While there's a long-term vision of having downstream commands be accepting of SHARD data as input, until such time, it might be more convenient for users if, at the command-line of dwimotioncorrect, they can request an alternative output that is the mssh2amp-transformed version of the output based on the gradient table of the unprocessed input image.