TST: add tests for estimate_sigma.
TST: add tests for estimate_sigma.

rjeli/scikit-image,paalge/scikit-image,paalge/scikit-image,vighneshbirodkar/scikit-image,rjeli/scikit-image,vighneshbirodkar/scikit-image,vighneshbirodkar/scikit-image,rjeli/scikit-image,paalge/scikit-image