Freeflow/cornervelocity

What this MR does / why does DuMux need it:

     * ------------
     * |     xxxx o
     * |     xxxx o
     * |     xxxx o
     * -----------*bbbbbbbbbbb
     * |     yyyy o zzzz     |
     * |     yyyy o zzzz     |
     * |     yyyy o zzzz     |
     * -----------------------

If a scvf (here o) touches a corner, at which there is a Dirichlet condition for face b (half-control volumes x, y and z), the transported velocity at * exists and is taken. No upwinding or higher-order approximation for the velocity at * are required. This MR intends to implement that behavior. Before, the velocity * was gotten by upwinding/tvd of velocity values above and below. This also means the transported velocity at * is now the same for the half-control volumes y and z and not, as previously, different.

Which issue this MR fixes: closes #890 (closed)