Provide backward-compatibility with dune-functions-2.6
I need to run dune-fufem with the 2.6 release of Dune, and because dune-fufem does not have a 2.6 release itself I need to use the master branch. Unfortunately, dune-fufem master has already stopped using Functions::LocalIndexSet, and therefore will not work with dune-functions-2.6.
This small merge request adds (some) backward-compatibility code to allow both dune-functions 2.6 and later.