Use some new range-based for loops

dune/fem-dg/algorithm/caller/sub/additionaloutput.hh

dune/fem-dg/algorithm/sub/elliptic.hh

dune/fem-dg/assemble/primalmatrix.hh

dune/fem-dg/examples/dataio/checkpointing.hh

dune/fem-dg/examples/stokes/stokesalgorithm.hh

dune/fem-dg/examples/stokes/stokesassembler.hh

dune/fem-dg/misc/dgnorm.hh

dune/fem-dg/operator/adaptation/adaptation.cc

dune/fem-dg/operator/adaptation/estimator.hh

dune/fem-dg/operator/adaptation/estimatorbase.hh

dune/fem-dg/operator/adaptation/poissonestimator.hh

dune/fem-dg/operator/adaptation/stokesestimator.hh

dune/fem-dg/operator/adaptation/utility.hh

dune/fem-dg/operator/dg/discretemodelcommon.hh

dune/fem-dg/operator/dg/fluxdiscretemodel.hh

dune/fem-dg/operator/fluxes/diffusion/dgprimalfluxes.hh

dune/fem-dg/operator/fluxes/diffusion/fluxbase.hh

dune/fem-dg/operator/fluxes/diffusion/ldgflux.hh

dune/fem-dg/operator/limiter/limiterdiscretemodel.hh

dune/fem-dg/operator/limiter/limitpass.hh