Make code compile with the 2.6 core modules

Some of the code in dune-fufem actually required version 2.7 of dune-geometry and dune-functions, even though the dune.module file states that 2.5 is enough. I want to build a downstream module (dune-gfe) with dune-2.6, and to that end this patch generalizes some code in dune-fufem to also work with version 2.6 of dune-geometry and dune-functions.