Yet another Menger sponge

Sorry I could not resist -- the algorithm is too beautiful. It scales with 3^3^iter ;-)
Have a look at the scad file.

If you forgot what a Menger sponge is, have a look at the [Wikipedia article](https://en.wikipedia.org/wiki/Menger_sponge).

I choose the parameters in such a way, that the smallest detail is a multiple of the nozzle diameter.

Be careful with iterations above three (3). Using four (4) took me 1h to render, the file size is 100Mb and it prints 1d7h. Five iterations will be 27h and 2.7Gb, etc..