A Menger cube is a fractal composed of subunits that resemble
a Rubik's-cubes with a hollow interior:
      _________
     /        /|
     ######### |
     # ## ## # |
     ######### |
     ###   ### |
     # #   # # |
     ###   ### |
     ######### |
     # ## ## #/
     #########

There are several ways to build them in moltemplate:
1) You can define each cube as a 3x3x3 array of smaller cubes, and then delete
   the 7 interior cubes using the "delete" command.  (Each smaller cube is a
   similar structure containing an array of 3x3x3 even smaller cubes...)
2) You can define each cube as a list of 20 smaller cubes corresponding to the
   cubes that would have remained after deleting the 7 interior cubes.

Method 1 is a little bit simpler, but method 2 is much more efficient because
it never has to create sub-cubes which will be deleted later.

This example uses method 2.




