Automatically fix orientation of volume meshes from gmsh?

Given the grief in https://github.com/illinois-ceesd/drivers_y2-isolator/issues/13, I wonder whether it might not be better to automatically fix the orientation of volume meshes coming in from gmsh.

Some aspects to consider.
- There's of course some cost to this.
- As of #299, the element orientation check is no longer disabled by `python -O`.
- Automatic flipping is only possible for volume meshes (AFAIK).
- Orientation finding and flipping is only implemented for simplicial meshes ATM. This is important because any automatic feature breeds an expectation that it works unconditionally, on all types of meshes. It's not hard to fix this though.

@lukeolson @mtcam @anderson2981, opinions?