Add divergence conforming discontinuous Galerkin demo for the Navier-Stokes equations

[jpdean]

This PR adds a divergence conforming discontinuous Galerkin demo for the Navier-Stokes equations.

It demonstrates:

• how to implement DG methods
• how to use predefined and custom restriction operators
• how upwinding can be implemented
• the use of H(div) elements in a non-conforming context

As far as I'm aware, none of the other demos demonstrate the above functionality.

I still need to so some tidying. Any feedback would be much appreciated.

[jpdean]

If anyone feels this would be better kept outside the main repository, it could be added to the FEniCSx tutorial instead if @jorgensd is happy