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.

skimage/morphology/misc.py

@@ -1,5 +1,6 @@
 import numpy as np
 import functools
+import warnings
 import scipy.ndimage as nd
 from .selem import _default_selem
 
@@ -128,6 +129,10 @@ def remove_small_objects(ar, min_size=64, connectivity=1, in_place=False):
                          "relabeling the input with `scipy.ndimage.label` or "
                          "`skimage.morphology.label`.")
 
+    if len(component_sizes) == 2:
+        warnings.warn("Only one label was provided to `remove_small_objects`. "
+                      "Did you mean to use a boolean array?")
+
     too_small = component_sizes < min_size
     too_small_mask = too_small[ccs]
     out[too_small_mask] = 0