Revise the WorkflowManager for more DataFrame Objects

[crantila]

I need to fix whatever breaks as a result of the wider use of DataFrame objects. As it goes, I may need to add more methods here.

• [x] _variable_part_modules()
• [x] _all_part_modules()
• [x] _two_part_modules()
• [x] _intervs()
• [x] _run_off_rep()
• [x] _run_freq_agg()
• [x] _remove_extra_pairs()
• [x] _get_dataframe() which will soon be named _filter_dataframe()
• [x] output()
• [x] _make_histogram()
• [x] export() which will soon be named _export()

[crantila]

I've removed _make_lilypond() because I'll just handle it later when I come back to pick up issue #301.