OpenSCADvent day 9 - Meissner Tetrahedra (and vec3 math tools)

There are a few Meissner tetrahedra SCAD models on Thingiverse already (and also a few models labeled "Messner" that are DEFINITELY NOT Meissner tetrahedra). However, I think this is the first one that does all the calculations in OpenSCAD, instead of deriving them externally and typing values in as magic numbers.

The only info this SCAD document starts with is the dihedral angle of a tetrahedron. Type in one vertex of the desired Meissner tetrahedron, and OpenSCAD does the rest!