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.