minor bug fix, allowing spaces to be an integer and not just a tuple with a single integer

src/operators/matrix_product_operator.py

@@ -98,7 +98,7 @@ class MatrixProductOperator(EndomorphicOperator):
             self._spaces = utilities.parse_spaces(spaces, len(self._domain))
             appl_space_shape = []
             active_axes = []
-            for space_idx in spaces:
+            for space_idx in self._spaces:
                 appl_space_shape += self._domain[space_idx].shape
                 active_axes += self._domain.axes[space_idx]
             appl_space_shape = tuple(appl_space_shape)