DOC: faster checking for c3d.
DOC: faster checking for c3d.

Former-commit-id: 423d1d7528b64354c107935958941523c3925fb5 [formerly 3db343e9090d088f4f6bfd606d2e96be04eb2071]
Former-commit-id: 6ff817f60446fa32d1b46562319a1d288b83d908
Former-commit-id: 8d9fcf63ec810644058b3d6878b4e8cadb93b2ba
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox