[freeflow] Introduce full shear stress terms, or document assumptions properly

We ran into a few discussions this summer/fall regarding terms that we may have either not implemented in the navier-stokes environment, or terms that we have neglected, but have not documented the assumptions properly.

These should be included, in either both new and old staggereds, or only the new staggered.

These terms include:

• the dilataion term

\tau = \mu (\nabla v + \nabla v^T) + ( \lambda \nabla \cdot v ) I

• with stokes hypothesis

(\lambda = 2/3 \mu)

<<< (MOVED TO RANS) Include the second term of the linear eddy viscosity reynolds stress: cfdOnline >>>

\tau_t = 2 \mu_t S - 2/3 \rho k \delta_{ij}

• any others?

From what I've seen, some of this is already underway. This issue is only a location to collect problems and track progress.