Feature request: Improved brain masking

[styner]

Allow users to select from a set of automated DWI brain masking tools:

• synthstrip on T2/baseline/B0 https://surfer.nmr.mgh.harvard.edu/docs/synthstrip/ (needs specification of appropriate FreeSurfer installation, similar to the current specification of FSL for bet)
• synthstrip on FA
• antspynet - brainextraction (FA for FA, alternatively either T2 or T2infant for baseline/B0)
• https://github.com/pnlbwh/CNN-Diffusion-MRIBrain-Segmentation
• allow for future other extensions