Add av diff

[anderson2981]

Extends dv and gas model to handle an additional term that represents extra diffusivity at species interfaces.

Questions for the review:

• [x] 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).
• [x] Is every top-level method and class documented? Are things that should be documented actually so?
• [x] Is the interface understandable? (I.e. can someone figure out what stuff does?) Is it well-defined?
• [x] Does the implementation do what the docstring claims?
• [x] 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?
  • See review comment.