Revert "update counting docstring for electron_count. Its best to use…

… a np.ndarray." I meant to add this to a feature branch.

This reverts commit 6ec823e.

python/stempy/image/__init__.py

@@ -201,9 +201,9 @@ def electron_count(reader, darkreference=None, number_of_samples=40,
 
     :param reader: the file reader that has already opened the data.
     :type reader: stempy.io.reader
-    :param darkreference: the dark reference to subtract. For an empty dark
-                              use np.zeros((Nx, Ny)).
-    :type darkreference: numpy.ndimage
+    :param darkreference: the dark reference to subtract, potentially generated
+                          via stempy.image.calculate_average().
+    :type darkreference: stempy.image.ImageArray or stempy::Image<double>
     :param number_of_samples: the number of samples to take when calculating
                               the thresholds.
     :type number_of_samples: int