A Python version of mcnp2cad

[pshriwise]

I've long had the sense that Cubit/Trelis' Python interface allows us to do virtually all of the same operations required here and with the emergence of @makeclean's csg2csg project, a Python port of this project would (in my mind) allow us to get direct CAD conversion from a variety of native formats. In theory, it could simplify the svalinn plugin build and install process as well (fewer libraries/depdendencies etc.)

Alternatively, this port could go directly into csg2csg or be added to that project with the Cubit/Trelis python interface as an optional dependency.

As it stands, we should be able to use csg2csg to go from some native format --> MCNP --> CAD, but this feels much more clean to me.

[pshriwise]

One hangup I just remembered is that the Trelis Python interface is still stuck on 2.7 (blerg), so maybe we table this discussion until that changes. Mostly wanted to get this down in writing or start a discussion about it somewhere.