Implement Navier-Stokes 3D

[wdeconinck]

• Depends on implementation of Navier-Stokes 2D ( issue Implement Navier-Stokes 2D #2 )
• Implementation analogous to Navier-Stokes 2D

[wdeconinck]

The files that should be implemented for 3D:

• cf3/sdm/equations/navierstokes/NavierStokes3D.*
• cf3/sdm/equations/navierstokes/Terms3D.*

The Terms3D need to implement 3 important functions:

• compute_convective_flux( state , normal )
• compute_riemann_flux( left_state, right_state, normal )
• compute_diffusive_flux( state, normal )

A few other functions to be implemented involve more
setting the correct things in the states and gradients of states.

Look at the 2D implementation. There I used some premade functions from the kernel library. However, you can just implement things immediately in the 3 above mentioned important functions.
Of course I will be available for help.