Make the PDElab istl backend of the 2.6 release work with the constified interface of dune istl 2.7
requested to merge michael.wenske/dune-pdelab:feature/makeISTLBackend2.7compatible into releases/2.6
Until there is a proper 2.7 release of PDElab, these changes should switch the const/non-const interface depending on whether dune-istl version is >=2.7 present, so that for the time being PDElab 2.6 can be used with istl 2.7 -and- 2.6 versions.
Edited by René Heß