Check that the reduced dim is an integer

lib/cartesian/Cartesian_red_black.h

@@ -176,7 +176,8 @@ public:
 
 	// Use a reduced simd grid
 	_simd_layout[d] = simd_layout[d];
-	_rdimensions[d]= _ldimensions[d]/_simd_layout[d];
+	_rdimensions[d]= _ldimensions[d]/_simd_layout[d]; // this is not checking if this is integer
+  assert(_rdimensions[d]*_simd_layout[d] == _ldimensions[d]);
 	assert(_rdimensions[d]>0);
 
 	// all elements of a simd vector must have same checkerboard.