Extra flags/attributes for OpNodes - Timing stats + Flags

* Added attributes for logging how long it took to evaluate an operation
  (and/or the last timestamp when it was evaluated)
* Added flag to tag operations which require the use of Python
  (there are implications for the scheduler here re-GIL threading lock issues
   but also our more general Py-Security concerns). See r58801 for example
   of threading issues...
Graph building code now tags drivers needing Python accordingly