Fix/box scvf construction

There has been a bug for the determination of the corners of boundary scvfs in the box scheme. This was probably never really tested but manifested itself in the implementation of the box-dfm scheme now...

Up to now, we were using info only from the intersection geometry, which could have a different ordering on a ug, alu or yasp grid. The work around now was to store a reference to the reference element in the helper class and reuse it when constructing the boundary scvfs. I tested it with Dune 2.5 and Dune 2.6. In the next Dune release, the alias for the reference elements would have to be adapted.