Add code to preprocess anatomy/fieldmap data

[jcohenadad]

To make sure we can all reproduce the results of this project, i suggest we put on GH the code that:

• fetches a specific version of the dataset
• loops through all subjects
• perform whatever preprocessing is needed (eg: smoothing of masks, related to Weird Artifacts in Masks #1 ), cropping (eg: based on vertebral labels to only keep the brain + c-spine)
• perform processing (eg: getting susceptibilty and fieldmap), related Weird Artifacts in Masks #1
• run shim layout optimization model @Govish

[mathieuboudreau]

Steps 1-4 listed are done as of v1.1 - https://github.com/shimming-toolbox/b0-fieldmap-realistic-simulation/tree/v1.1