Refactor: WorkflowManager._get_dataframe() work with DataFrame as input

Previously the method wouldn't work properly if WorkflowManager._result was
already a DataFrame. Now it does.

Working toward #321
Refactor: WorkflowManager._make_histogram() uses barchart.RBarChart

Previously, _make_histogram() would call the R script by itself. Now, it uses
the RBarChart experimenter to "outsource" that issue.

Closes #321
Remove a TODO that's already done