Refactor: WorkflowManager._interval_ngrams() for VIS 2

The changes are very minor, mostly for documentation. But I also removed an
unnecessary call to ast.literal_eval().

Related to #325
Refactor: WorkflowManager._run_off_rep() for VIS 2

Basically, this removes that method's ability to accept and output a dictionary
that preserves the ordering of keys. We don't need that in VIS 2.

Related to #325
Minor fixup in WorkflowManager._run_freq_agg()
Refactor: WorkflowManager._variable_part_modules() for VIS 2

Now with simpler tests!

Related to #325
Refactor: WorkflowManager._two_part_modules() for VIS 2

It works.

Related to #325
Refactor: WorkflowManager._all_part_modules() for VIS 2

It works now too.

Related to #325
Fixed: Incorrect kwarg in some WorkflowManager tests

I was using the 'autospec' kwarg to MagicMock() instead of the 'spec_set' kwarg.
The former doesn't exist, so it didn't really work. Now it's fixed.
Refactor: WorkflowManager.output() for VIS 2

This method and its tests didn't *need* any changes for VIS 2, but I updated
them anyway. What better time is there to mess around with things than RTFN?

Related to #325
Refactor: WorkflowManager._get_dataframe() ==> _filter_dataframe()

Now it'll work for VIS 2 too! It performs three types of filtering: the
usual "top x" and "threshold" filter, plus a renaming filter that will
be useful for running the R bar chart script.

Related to #325
Refactor: WorkflowManager._make_histogram() for VIS 2

Really, just refactoring for the changes in _filter_dataframe()

Related to #325
WIP-Refactor: WorkflowManager._make_lilypond()

It's not finished yet, but it's going to take a lot of work, so I'm sort of
just saving my half-complete results and I'll come back to it.

Related to #325 and #301
Refactor: WorkflowManager.export() for VIS 2

It works. The rename as per issue #317 is still pending.

Related to #325
Refactor: Minor revisions suggested by pylint

You know what? It's right. Sometimes my code is sloppy. But sometimes also
it tells me I'm not supposed to access my unit-under-test, and then I'm all
like, "yo pylint, disable=protected-access."