add cube example to the demos

demos/cube.geo

+lc = 0.3;
+Point(1) = {0.0,0.0,0.0,lc};         
+Point(2) = {1,0.0,0.0,lc};         
+Point(3) = {1,1,0.0,lc};         
+Point(4) = {0,1,0.0,lc};         
+Line(1) = {4,3};         
+Line(2) = {3,2};         
+Line(3) = {2,1};         
+Line(4) = {1,4};         
+Line Loop(5) = {2,3,4,1};         
+Plane Surface(6) = {5};         
+Extrude Surface { 6, {0,0.0,1} };         
+
+// try netgen:
+// Mesh.Algorithm3D = 4;