Desmod

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* [FIX] Join worker processes in simulate_many()
* [FIX] Ensure PriorityQueue's items are heapified

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* [NEW] Add config_filter param to simulate_factors() (14)
* [FIX] Use pyenv for travis builds

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* [NEW] Add desmod.dot.generate_dot()
* [NEW] Add "persist" option for tracers
* [NEW] Add SQLiteTracer
* [NEW] Add grocery store example
* [NEW] Support probing a Resource's queue
* [FIX] Stable sort order in DOT generation
* [CHANGE] Rearrange doc index page
* [CHANGE] Change examples hierarchy
* [CHANGE] Add DOT to Gas Station example
* [CHANGE] Tests and cleanup for desmod.probe

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* [CHANGE] meta.sim.index and meta.sim.special
* [CHANGE] Add meta.sim.workspace
* [FIX] Check simulate_many() jobs
* [CHANGE] Add named configuration categories and doc strings

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* [CHANGE] Make NamedManager.name() deps argument optional
* [FIX] Add test for desmod.config.parse_user_factors()
* [FIX] More testing for tracer.py

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* [FIX] Documentation repairs for desmod.config
* [FIX] Add tests for sim.config.file
* [FIX] Annotate no coverage line in test_dot.py
* [NEW] Add desmod.config.apply_user_config()
* [NEW] Support dumping JSON or Python config and result