RF: Make idx a positional argument.

You probably always pass an index when you are trying to get an item.

Author: arokem

nibabel/spatialimages.py

@@ -656,7 +656,7 @@ def from_image(klass, img):
                      klass.header_class.from_header(img.header),
                      extra=img.extra.copy())
 
-    def __getitem__(self, idx=None):
+    def __getitem__(self, idx):
         ''' No slicing or dictionary interface for images
         '''
         raise TypeError("Cannot slice image objects; consider slicing image "