OPT: average_data_within_mask: now computes weighted STD (issue #429)
OPT: average_data_within_mask: now computes weighted STD (issue #429)

Former-commit-id: 9fdf03e81d5a5857a7bcbd4dcb13cc23522afca8
Former-commit-id: a21404a9c08a532ec7d45f89cce9c819a333d153
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox