OPT: add flags for noise background threshold and std manual
OPT: add flags for noise background threshold and std manual

Former-commit-id: 76372c4c2e9a4afedb88c49eb041b6ca3de5f3f1 [formerly 1102de74dc3b3617fa86144cc7d07fd2afd70a80]
Former-commit-id: a92177bf765a1aaefc7590ee62fc463fa7aa0985
Former-commit-id: dc91945b59c6197be6015fc87200f5adb00f3e89
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox