[grid.walker] added files from dune-gdt
[grid.walker.wrapper] use correct type
[grid.walker] moved ApplyOn to separate header
[profiler] synchronize startTiming
[cmake] adds TBB discovery
[cmake] cleanup obsolete finds
[common.crtp] added non-static variants of as_imp()
[tests] adds one for Walker
[cmake] fixes tbb debug mode linking