support for triggers within workflows, and various improvements

* Added support for cron and notice triggers in workflows:
  configuration, execution and graphviz diagram
* Added parameters to notices (when posted)
* Implemented parameters overriding from requesttask actions
* Reject task definition as invalid every time an error-level
  message is logged. Formerly there were many cases where
  tasks where configured anyway, with probable errors.
* Added 3 workflow tasks in massive2b.conf sample to be used
  for endurance and regression testing
* Removed redundant instance/_instance all over Executor's
  xxxMean() methods
* Plus some minor fixes and enhancements