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

• turbine_model_t - abstract interface for turbine forcing models, with methods init, update, project_forces, write_output. Ships with a no-op dummy (turbine_dummy_t, also the iturbine=0 model) so the case runs before any real model exists.
• wind_turbine case - uniform inflow / convective outflow in x, using a polymorphic turbine_model_t. The case owns the turbine object and is the only place the iturbine switch appears (at allocation time); after that it calls the polymorphic turbine with no branching.
• iSVV / hyperviscous dissipation - added c_nu and nu0_nu for the compact6-hyperviscous scheme. This is the implicit LES the two_turbines case uses ( Incompact3d jles=4) - implicit numerical dissipation, not an explicit SGS model (the explicit Smagorinsky LES issue is Add Smagorinsky LES with wall damping #321). These params are optional.
  • Boundary conditions - Dirichlet / periodic / periodic (matches the cylinder case). In Incompact3d the example uses free-slip walls in y and z, but the FFT Poisson solver currently requires z periodic and supports only a single MPI rank in non-periodic directions.

Closes #309