added Arc to vmath; small fixes; extend documentation;

Hi Bryan,

The biggest chunk of work in this pull request is the vmath/arc.py code: it is a helper class to resolve arc geometry and calculate unknown arc elements based on the known ones.

Beside that, I added a stub for triangle geometry, for one method I think it belongs there, but didn't implement the object itself yet (the arc I need now, the triangle not yet).

There are some ascii-art documentation additions too, I'm not sure how useful but was fun to do it :-)

The cached _bindings fixes are only useful while doing development, to avoid generation of the brlcad bindings if that part of the code didn't change and I want a quick deploy (it can take quite some time to generate the bindings on my machine).

Cheers,
Csaba