Add gas tester

[MTCam]

Adds gas-in-box.py example that is very convenient for testing code and model features. Also adds some thermochemical mechanisms useful for testing the effect of increasing nspecies, and changing mechanisms.

Questions for the review:

• [ ] Is the scope and purpose of the PR clear?
  • [ ] The PR should have a description.
  • [ ] The PR should have a guide if needed (e.g., an ordering).
• [ ] Is every top-level method and class documented? Are things that should be documented actually so?
• [ ] Is the interface understandable? (I.e. can someone figure out what stuff does?) Is it well-defined?
• [ ] Does the implementation do what the docstring claims?
• [ ] Is everything that is implemented covered by tests?
• [ ] Do you see any immediate risks or performance disadvantages with the design? Example: what do interface normals attach to?

[tulioricci]

Would it be easy/possible/useful to output DAG information in this driver?

[MTCam]

Would it be easy/possible/useful to output DAG information in this driver?

I use it for that, and for testing lots of different stuff like free-stream preservation with TPEs. It is essentiallly a more configurable combozzle.