Merge branch 'feature/reference-elements-by-value' into 'master'

dune/fufem/assemblers/transferoperatorassembler.hh

dune/fufem/boundarypatch.hh

dune/fufem/boundarywriter.hh

dune/fufem/dgindexset.hh

dune/fufem/estimators/errorfractionmarking.hh

dune/fufem/facehierarchy.hh

dune/fufem/functions/alienelementlocalbasisfunction.hh

dune/fufem/functions/deformationfunction.hh

dune/fufem/functionspacebases/p3nodalbasis.hh

dune/fufem/functiontools/p0p1interpolation.hh

dune/fufem/quadraturerules/lumpingquadraturerule.hh

dune/fufem/referenceelementhelper.hh

dune/fufem/test/subgridxyfunctionalassemblertest.cc

dune/fufem/utilities/hierarchicleafiterator.hh