REF: recoded sct_estimate_MAP_tracts into sct_estimate_tracts. Code is lighter. Still need to add old functionalities.
REF: recoded sct_estimate_MAP_tracts into sct_estimate_tracts. Code is lighter. Still need to add old functionalities.

Former-commit-id: 9a0c209c1352ec99617b6c26c6bf80a5758cd91e
Former-commit-id: 8b9624a928e52adc5a87c73da5a18567485e0b32
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox