`./csgtool audit mesh` command

This is work to follow on the work in #57 to address the noise issues in meshes after operating on them. The work here should help test the work there, and see if the proposed solution is solving an actual problem.

### Planned mesh audit checks

  * [x] Report number of zero-length polygon edges
  * [x] :question: :grey_question: -- Whatever else might come up as a flaw that can be detected.