Fractal generator

Based on axioms and generators the function generates fractal structures of a specified level and one of the predefined geometries (Sierpinski's triangle and pyramide and Menger's sponge).

Works in two ways: 1) Generation of one of the three
predefined geometries or 2) Alternative the axiom and generator coordinates may be specified to generate other fractal than those predefined.

Usage (for predefined use):
fractal(N,s);
where N is the level and s=1/2/3

Examples:
fractal(2,1); % show the Sierpinski's triangle with 2 levels.
fractal(0,3); % show the axiom of the Menger's sponge.
fractal(1,3); % show the generator of the Menger's sponge.