[pnm] BoundaryFlux class only works for Dirichlet Boundaries

The BoundaryFlux class for evaluating fluxes entering or leaving the network only works if Dirichlet conditions are set on the relevant boundaries.

This is because the class evaluates the local residual at the respective DOFs. For Dirichlet BCs, the value is not zero but equals the boundary flux.

One could maybe check if a Dirichlet BC is set and proceed as before and just evaluate the source term instead otherwise.

Note: source corresponds to a neumann BC at the boundary. The PNM does not support neumann but requires a source instead because there are no boundary scvfs.