[staggered][freeflow] Fix handling of Neumann BCs

* fluxes over all faces (except the one on the boundary)
  need to be considerd

dumux/freeflow/navierstokes/staggered/fluxvariables.hh

@@ -248,7 +248,7 @@ public:
         // Handle inflow or outflow conditions.
         // Treat the staggered half-volume adjacent to the boundary as if it was on the opposite side of the boundary.
         // The respective face's outer normal vector will point in the same direction as the scvf's one.
-        if(scvf.boundary())
+        if(scvf.boundary() && problem.boundaryTypes(element, scvf).isDirichlet(Indices::pressureIdx))
             frontalFlux += inflowOutflowBoundaryFlux_(problem, element, scvf, elemVolVars, elemFaceVars);
 
         // Account for the staggered face's area. For rectangular elements, this equals the area of the scvf

dumux/freeflow/navierstokes/staggered/localresidual.hh

@@ -299,7 +299,7 @@ protected:
                 residual = problem.neumann(element, fvGeometry, elemVolVars, elemFaceVars, scvf)[Indices::velocity(scvf.directionIndex())]
                                            * extrusionFactor * scvf.area();
 
-                // treat the remaining (normal) faces of the staggered control volume
+                // treat the remaining (frontal and lateral) faces of the staggered control volume
                 residual += computeFluxForFace(problem, element, scvf, fvGeometry, elemVolVars, elemFaceVars, elemFluxVarsCache);
             }
             else if(bcTypes.isSymmetry())