GMR 287: Support for AthSequencers by Scheduler
See gaudi/Gaudi!287 (merged) (not yet accepted)
This adds support for ATLAS's AthSequencers to the scheduler, so that it can do proper Control Flow.
It also removes the explicit class "GaudiAtomicSequencer", and replaces it with an "Atomic" Property in the GaudiSequencer (and AthSequencer) classes.