Radical.pilot

- support for bulk callbacks
- fixed package paths for launch methods
(radical.pilot.agent.launch_method)
- updated documentation references
- raise minimum Python version to 3.6
- local submit configuration for Frontera
- switch frontera to default agent cfg
- fix cray agent config
- fix issue 2075 part 2


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- fix dependency version for radical.utils


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- code cleanup


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- transition to Python3
- migrate Rhea to Slurm
- ensure PATH setting for sub-agents
- CUDA is now handled by LM
- fix / improve documentation
- Sched optimization: task lookup in O(1)
- Stampede2 prun config
- testing, flaking, linting and travis fixes
- add `pilot.stage_out` (symmetric to `pilot.stage_in`)
- add noop sleep executor
- improve prrte support
- avoid state publish during idle times
- cheyenne support
- default to cont scheduler
- configuration system revamp
- heartbeat based process management
- faster termination
- support for Frontera
- lockfree scheduler base class
- switch to RU ZMQ layer


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- port pubsub hotfix


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- transition to Python3


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