added test for post calls to dpc
opened up /dpc url for post requests
re-purposed DPC model as a wrapper around a JSON file
implemented functional POST behavior for DPC API
renamed test_pgsql_installation file
applied small fixes to dpc model, updated its test
also renamed the other tests
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
Merge pull request #188 from bepetersn/v2.0-dev

VS 2.1.1 -- created one possible version of the DailyPopulationChanges model; integrated with GET and POST
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