feat(tree-pilled): feature additions: enabled realistic testing of tree behavior, concretized testing schema, fixed bugs in tick engine, squared away some docker things

[joshc-slac]

Changes

• added dockerfile based on this image, for a defined rhel7 testing environment for things BEAMS
• added some documentation specifically around BEAMS specific concepts
• Changed Sequencer message handler to be a python threading object as opposed to a Worker (multiprocessing), there were some non trivial to circumvent issues with the GIL
• Fixed a "bug" in the tick engine
• Created beams/tree_generator/TreeGenerator.py which will serve to hold all of our dynamic tree behavior
• will clean up commit on top of this with better documentation