Two aligned wind turbine example case

[ia267]

Created the flow case for the wind turbine example, including an interface for the turbine models (to be implemented later and only contains a dummy model for now).

changes

• added c_nu and nu0_nu for compact6-hyperviscous. This is the LES model for the turbune case. These params are optional
• turbine_model_t - interface with methods init, update, project_forces, write_output
• wind_turbine case - uniform inflow/convective outflow using poloymorphic turbine_model_t

Boundary conditions

• Dirichlet / periodic / periodic (matches the cylinder case). In Incompact3d, the example uses free-slip walls in y and z (currenty FFT Poison solver requires z periodic and only supports a single MPI rank for non-periodic directions).

Closes #309