[staggered][faceVariables] Retrive boundary values in fluxVars

* get the values for parallel and normal velocities directly in the
  fluxVars and not during the update of the faceVars
* this is necessary in the context of coupling when the velocities on the
  boundary shall be inferred from the other domain
* the gridFaceVars  are initialized (and thus updated)
  before any coupling context can be bound so this update step
  must not depend on some coupled context