WIP: [tpfa] Add possiblity for custom boundary function

I needed to mark certain intersections as boundaries although they are not boundaries in the grid (i.e. have a neighbor). This can be for example a touching point/edge for networks that should be marked as a boundary. One possibility is to modify the grid to double the vertex/edge. Another possibility is implemented in this MR and is providing a function that determined which intersections are boundaries. Rough implementation, can be improved. For example, I don't think the function actually has to be stored. It's enough to pass it to update I think.

• Remove debug output
• pass function to update