compatibility fix for asl, part-comb, and quantitative maps

fixes for compatibility with Maastricht data (asl, combined phase, and qmaps)

major bugfix for processing phasediff fieldmap images

• this version corrects a bug that mangled the *_phasediff field map images
  • phasediff images were incorrectly being post-processed as if they were magnitude images
• this fix does not affect you unless you are using phasediff field maps

skip already corrected images

• added check to skip images that have already been processed by this app
  note: re-release after minor circleci fix

intermediate files in sourcedata/

• moved storage of intermediate files in the output dataset from derivatives to sourcedata

fix for headers and improved efficiency

• fixes issues with modified headers that caused some downstream apps to fail
• improved efficiency by not re-running gradunwarp for images with same FOV (using hash on sform)
  -circleCI build (no tests yet! - need to create a fake grad-coeff to test)