Replace NULL with nullptr
Remove currently unused csb_tree
Merge pull request #238 from bastih/master

Replace NULL with nullptr
Merge pull request #239 from bastih/master

Remove currently unused csb_tree
Fix some issues surrounding WITH_PAPI
Merge pull request #240 from bastih/master

Fix some issues surrounding WITH_PAPI
Using weak pointers instead of bare pointers.

	This fixes the problem that tasks might be deleted while they are still running notifyReadyObservers. Since observers did not declare any ownership of the "to be notified" tasks, this was bound to happen. Weak pointers seem a good fit in this case since shared pointers would create circular references and tasks would never be deleted.