prints warning if likwid is used with instrumentation level 3

I've already mentioned this problem in [!314] but I think more
visibility for this problem is useful

python/dune/codegen/pdelab/driver/timings.py

@@ -133,6 +133,9 @@ def setup_timer():
         include_file("likwid.h", filetag="driver")
         init_likwid()
         if get_option('instrumentation_level') >= 3:
+            import logging
+            logger = logging.getLogger(__name__)
+            logger.warning("timings: using instrumentation level >= 3 with likwid will slow down your code considerably")
             local_operator_likwid()
         finalize_likwid()
     else: