Marked functions that are too general for this module

Functions for splitting or joining object_ids, guessing model
type from the filename, etc, etc.  These should be moved to
DDR/models.py, DDR/__init__.py, or a new module.
removed unused functions
change arg name to be consistent with other functions
replace globals or settings with explicit arguments

Instead of using global variables to refer to location of mapping,
facet, and model JSON files, pass them in as arguments. Will make
testing easier.
added @params, usage examples, and more documentation