Projections for ns3d solvers

The goal would be to be able for ns3d solvers to solve modified equations with different geometric projections of the velocity field in Fourier space.

It is somehow similar to what we do with the parameter no_vz_kz0, so one can look for this string in the code to get inspiration.

In particular, I think we should create a new parameter for ns3d solvers (I guess here) and apply projections in the tendencies_nonlin methods (in particular for ns3d and ns3d.strat).

I'm going to open a PR to see how this can be done.