New slip boundary condition for IncNS

[nfehn]

To prescribe slip BCs with vanishing wall-shear stress for the module IncNS, we currently have the boundary condition of type Symmetry.

However, in the Euler case with vanishing viscosity nu=0, one might argue that vanishing wall shear stress does not imply that one wants to set certain components of the velocity gradient to zero at the wall as in the viscous case with positive viscosity. In order to just prescribe u*n=0 (i.e. no flow in direction normal to the boundary) and nothing for the gradients., I think we would have to introduce another BC of type Slip. Do you agree @kronbichler?