Parametric Tetrapod (including hollow)

This is a generic tetrapod with the option to generate a hollow version which I'm using for shrimp aquarium decoration.

There is a tricky bridge in this; print accordingly. I found that printing on a leg (three legs in the air) was always too top-heavy even with an aggressive brim; printing on the three-legs-down face with good support worked best.

In the OpenSCAD file I've included the code to generate just a single leg (`generateHollowTetraLeg()`); this is hacky and generally bad. It was from a misguided attempt to print four legs and join them. I don't think anyone should use this approach (my method of removing artifacts from leg intersection subtraction is *bad*), but I've included my prior art in case someone feels inspired or figures out a way to make it work.