updated dpc model initialization to require a pathname
updated GET and POST calls to invoke DPC model correctly
made DPC API tests more complete, added setup/teardown
setup default files to be used for DPC model for testing, production, and local usage; AND fixed circular import problems that ensued
pushed dpc path selection into flask config and env vars
added error message to help posterity
standardized data formatting methods for DPC model
added back in dpc model test with multiple data
change underlying data format from json to csv; ALSO implemented dynamic formatting methods for model
Merge pull request #189 from bepetersn/v2.0-dev

VS 2.1.1 -- DailyPopulationChanges (DPC) model, POST API, tests for both