remove get/get_obj dichotomy

First pass, have not done extensive testing. @hhuuggoo, @mattpap comments welcome. 

This is a pre-req for object selection.
i think we should extend to handle dicts.

There are some cases (I think column_refs) where we nest [(colname, datasource_ref), (colname, datasource_ref)].  I think we should revise our json model to flatten that out so we don't have to do recursive traversal..  Though - I believe that's only present in the data range 1d and we should just get rid of that anywyas