Draft: Adding a wall model for large-eddy simulations

When running implicit large-eddy simulations in Nektar++, the resolution on the boundary reaches the resolution of direct numerical simulations. The small first layer height puts a severe restriction on the time step size.

To overcome this problem, this merge request introduces a wall model which is based on the log law velocity profile of the turbulent boundary layer. The slip-boundary type wall model is used and the Robin boundary condition is treated as a Dirichlet condition.

The formulas are

in short u = lp * (tau - n tau \cdot n)