cubemeasure: mean and sum pros in base classes

src/cr/cube/matrix/cubemeasure.py

@@ -109,6 +109,13 @@ def factory(cls, cube, dimensions, slice_idx):
             dimensions, cube.means[cls._slice_idx_expr(cube, slice_idx)]
         )
 
+    @lazyproperty
+    def means(self):
+        """2D np.float64 ndarray of cube means."""
+        raise NotImplementedError(  # pragma: no cover
+            "%s must implement `.means`" % type(self).__name__
+        )
+
 
 class _CatXCatCubeMeans(_BaseCubeMeans):
     """Means cube-measure for a slice with no MR dimensions."""
@@ -182,6 +189,13 @@ def factory(cls, cube, dimensions, slice_idx):
             )
         return CubeSumsCls(dimensions, cube.sum[cls._slice_idx_expr(cube, slice_idx)])
 
+    @lazyproperty
+    def sum(self):
+        """2D np.float64 ndarray of cube sum."""
+        raise NotImplementedError(  # pragma: no cover
+            "%s must implement `.sum`" % type(self).__name__
+        )
+
 
 class _CatXCatCubeSums(_BaseCubeSums):
     """Sum cube-measure for a slice with no MR dimensions."""