Update msct_multiatlas_seg.py
Update msct_multiatlas_seg.py

REF: comment about the mode weight for the similarity copmutation
Former-commit-id: 595d376e7570830682dcf2105b1f2b9d581a12c5 [formerly 40f82fdc3a6bc3e0ff7f2e174376e949aa7ef63e]
Former-commit-id: 72553b44bfd35e42cda0b4603e0b2aeb0923c289
Former-commit-id: dc5110a28fb3a886ae189da4cebbbed73664325d
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox