Extension of Quail for handling diffusion terms

[bornhoft]

This pull request includes the addition of handling second-order terms in DG via the SIP discretization. This includes the following:

• Addition of the SIP diffusion flux in physics/base/functions.py
• Addition of the constant advection diffusion classes (with examples)
• Addition of the Navier-Stokes physics class (with example)
• Addition of interior diffusion fluxes for both classes above
• Addition of tests for NS diffusion flux
• Addition of transport types in navierstokes/tools.py (currently supports constant and Sutherland's law)
• Added the exact flux for linear advection
• Addition of multiple end-to-end tests
• Removed multiple if statements and cleaned up the main residual functions a bit

I have perhaps missed some of the changes here but I believe we have most things covered. Convergence tests have been completed for 1D/2D constant advection diffusion on both DG/ADER and on quads/tris for DG. Convergence tests have also been completed for 2D manufactured solution for DG (and almost completed for ADERDG).

Please feel free to reach out with more specific questions or comments. I believe that my list includes most of the major changes but this is a reasonably sized pull request so I imagine I have missed a few. Thanks for the extra set of eyes!

[codecov-commenter]

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