EnsembleFrame Refactor Milestone 4: Refactor Ensemble logic

[dougbrn]

#191 Milestone 4 focuses on the refactor of existing Ensemble (and test) logic into the new scope of the Ensemble and EnsembleFrame objects. Refactor the Ensemble logic tied to the object table and source table into logic tied to the ObjectFrame and SourceFrame. Do this probably one or a few functions at a time, and modify unit tests until passing.

Most of this will involve the removal of wrapped Dask API code and the migration of Ensemble dataframe functionality into the EnsembleFrame class.