[bug] fix octave failed test

Author: fangq

elemvolume.m

@@ -27,7 +27,9 @@
 edge2 = v3 - v1;
 
 if (size(elem, 2) == size(node, 2))
-    vol = 0.5 * vecnorm(cross(edge1, edge2, 2), 2, 2);
+    det12 = cross(edge1, edge2);
+    det12 = sum(det12 .* det12, 2);
+    vol = 0.5 * sqrt(det12);
     return
 end
 

vol2surf.m

@@ -107,6 +107,8 @@
                 else
                     regions = seeds;
                 end
+            else
+                regions(end + 1, :) = mean(levelno);
             end
         end
     end