ENH: Template (Atlas) workflow.  Output has been visually inspected and verified.

The output is:
  Atlas/            - copy of needed filed from Baseline results dir
    definitions/    - AtlasDefinitions_<subject>.xml files
  SUBJECT_TEMPLATES/
    <subject>/      - AVG_*.nii.gz files
      iteration1/   - AVG_T1.nii.gz (from first iteration)
ENH: Phase2
ENH: Longitudinal (Phase 2) workflow complete and visually inspected
WIP: Cherry-pick of longitudinal.py from 8717eb0