Add a compressible Navier-Stokes model

Create a pull request to do the following:

- Add a compressible Navier-Stokes operator, analogous to (and perhaps building on) the existing Euler one.
- Use Bassi-Rebay 1 (https://doi.org/10.1006/jcph.1996.5572) fluxes.
- Keep the code dimension-independent
- Use https://github.com/tcew/nodal-dg/blob/master/Codes1.1/CFD2D/CurvedCNSRHS2D.m as a guide.
- Allow for inflow, outflow, and no-slip boundary conditions.
- Add a convergence test for Poiseuille flow

cc @kaushikcfd