- Implementation of post-event loop tasks (Mihaela Gheata)

- Implementation of post-event loop tasks (Mihaela Gheata)
Post-event loop tasks are tasks for which the Exec() method is not called in the
main event loop, but afterwards. An example is fitting a histogram produced by
other task in the event loop. To use a task in this mode one has to:
1. Connect the task input(s) at container used as output(s) by normal event loop
task(s)
2. Use the method AliAnalysisTask::SetPostEventLoop(kTRUE), e.g.
   myFitTask->SetPostEventLoop(kTRUE);
When containing post event loop tasks, the analysis will run in 2 stages: the
event loop triggered by AliAnalysisSelector first, then all remaining post event
loop tasks.