Extra check that failed in quenched test

lib/cartesian/Cartesian_red_black.h

@@ -142,7 +142,10 @@ public:
 	// Use a reduced simd grid
 	_simd_layout[d] = simd_layout[d];
 	_rdimensions[d]= _ldimensions[d]/_simd_layout[d];
-	
+
+	// all elements of a simd vector must have same checkerboard.
+	if ( simd_layout[d]>1 ) assert((_rdimensions[d]&0x1)==0); 
+
 	_osites *= _rdimensions[d];
 	_isites *= _simd_layout[d];