Require tifffile

No matter what we need a copy of `tifffile` to process and convert TIFF
data. We optionally used the vendored copy of `tifffile` from
`scikit-image` previously as this might be an easier way for end users
to meet this requirement as it was more generally available. As
`tifffile` is available from `conda-forge` and we use that channel for
basically all of our dependencies, it doesn't make sense to not use
`tifffile` from there as well. This way we have a better chance of
having the newest version of `tifffile` available. As `tifffile` is
under the BSD 3-clause license, there are no additional constraints from
what we already have with `scikit-image` and they are all quite
reasonable.

setup.py

@@ -65,6 +65,7 @@ def run_tests(self):
         "xnumpy",
         "scikit-image",
         "scikit-learn",
+        "tifffile",
         "yail",
         "mahotas",
         "vigra",