Update mean's variables to match other functions (#73)

As the input image has been summed over, it doesn't really make sense to
include that in the variable names in `mean`. This same rational is used
in other functions. So update `mean`'s variable names accordingly.

Also fix the `com_lbl` name for the result variable in `mean` in the
process. This was an old copy-paste error from the `center_of_mass`
function, which makes no sense in the context of `mean`. It is now
`mean_lbl`, which is more fitting.

dask_image/ndmeasure/__init__.py

@@ -446,16 +446,16 @@ def mean(input, labels=None, index=None):
         input, labels, index
     )
 
-    input_sum = sum(input, labels, index)
-    input_norm = sum(
+    sum_lbl = sum(input, labels, index)
+    norm_lbl = sum(
         dask.array.ones(input.shape, dtype=input.dtype, chunks=input.chunks),
         labels,
         index
     )
 
-    com_lbl = input_sum / input_norm
+    mean_lbl = sum_lbl / norm_lbl
 
-    return com_lbl
+    return mean_lbl
 
 
 def median(input, labels=None, index=None):