message: make array[mask] += value work in Python
diff: @@ -420,8 +420,7 @@ def op_getitem(self, index):
  result[i] = op_getitem(self, indices[i])
  return result
  elif _ek.is_mask_v(index):
  - raise Exception("Indexing via masks is only allowed in the case of "
  - "assignments, e.g.: array[mask] = value")
  + return self
  else:
  raise Exception("Invalid array index! (must be an integer or a tuple "
  "of integers!)")
  