Merge branch 'master' into refactor-walker-functors

# Conflicts:
#	dune/xt/grid/exceptions.hh
#	dune/xt/grid/test/builder_definitions.cmake
#	dune/xt/grid/test/compiles_totals.pickle
#	dune/xt/grid/test/walker.cc
#	dune/xt/grid/walker.hh
#	dune/xt/grid/walker/apply-on.hh