Make PDELab compatible with Dune 2.6
This MR removes all the warnings/errors introduced by the 2.6 release and removes old stuff. I separated all the changes into separate commits. It may be more insightful to browse the commit list here, as some changes appear in 100+ places...
Tested with MPI, GMP, dune-alugrid, SuiteSparse, SuperLU, ArpackPP, ParMETIS, Eigen3 and dune-uggrid installed, with gcc 6.3.0 and clang 3.8.1. I also adapted and tested dune-pdelab-tutorials to find more stuff.
Edited by Dominic Kempf