istlbackend.hh: adapt to changes in dune-functions
requested to merge burchardt_at_igpm.rwth-aachen.de/dune-fufem:bugfix/dune-functions-changes into master
This isn't a proper solution as it uses implementation details out of
the Dune::Functions::Imp namespace and in some cases one might have
to use shiftedStaticMultiIndex, but it is better than before.
See also #19 (closed)