[Navier Stokes] Possibly revise initial condition of instationary channel test

I would say that the initial conditions of an instationary test should such that the Navier-Stokes and continuity equation can be fulfilled. Currently, we start our channel test with a velocity profile u=1 at the left boundary, u=0 everywhere else. This means that for the cells on the very left the continuity equation can not be fulfilled. @nedc had the idea to start with u=1 everywhere besides at the top and bottom boundary where we have u=0. This would be in accordance with continuity.