remove quaternions from stroke

improve docs for skin
cumprod multiplies on the left
echo_matrix bugfix

tests/test_math.scad

@@ -619,8 +619,8 @@ module test_cumprod(){
                        ]),
                        [
                         [[1,2],[3,4]],
-                        [[8,13],[12,31]],
-                        [[124,15],[232,57]]
+                        [[11,12],[18,28]],
+                        [[45,24],[98,132]]
                        ]);
   assert_equal(cumprod([[[1,2],[3,4]]]), [[[1,2],[3,4]]]);
 }