Warn users of `remove_small_objects` about type

When input type is `int`, we assume that the image is pre-labeled.
However, when only 0 and 1 are present, it may be that the user expects
us to treat it as a binary image. We don't, but now issue a warning
that they might want to use a bool image.

Fixes #1178.
Test type warning from remove_small_objects
Merge pull request #1183 from jni/warn-single-labels

Warn when single int label of 1 is passed to remove_small_objects