Sets the flux in neumann_t to be an array instead of a scalar.
Adds shear_stress_t, which wraps 2 neumann_t and a dirichlet_t and allows setting a given stress in the x direction. Sets 0 stress in z and 0 velocity in y.
Adds a wall_model_bc_t, which descends from shear_stress_t and uses a wall model to compute the stress.
Cherry-pick from stress_formulation.