Fall back on .vindex when basic indexing fails

Addresses zarr-developers#657

This matches NumPy behaviour in that basic, boolean, and vectorized
integer (fancy) indexing are all accessible from `__{get,set}item__`.
Users still have access to all the indexing methods if they want to be
sure to use only basic indexing (integer + slices).

Author: jni

zarr/core.py

@@ -658,8 +658,12 @@ def __getitem__(self, selection):
 
         """
 
-        fields, selection = pop_fields(selection)
-        return self.get_basic_selection(selection, fields=fields)
+        fields, pure_selection = pop_fields(selection)
+        try:
+            result = self.get_basic_selection(pure_selection, fields=fields)
+        except IndexError:
+            result = self.vindex[selection]
+        return result
 
     def get_basic_selection(self, selection=Ellipsis, out=None, fields=None):
         """Retrieve data for an item or region of the array.
@@ -1209,8 +1213,11 @@ def __setitem__(self, selection, value):
 
         """
 
-        fields, selection = pop_fields(selection)
-        self.set_basic_selection(selection, value, fields=fields)
+        fields, pure_selection = pop_fields(selection)
+        try:
+            self.set_basic_selection(pure_selection, value, fields=fields)
+        except IndexError:
+            self.vindex[selection] = value
 
     def set_basic_selection(self, selection, value, fields=None):
         """Modify data for an item or region of the array.