Change API Theme to sphinx_rtd_theme

So slick, all the flat, cool theme, wow.
Revisions to NGramIndexer

- change u'mark singles' setting to u'mark_singles'
- update tests to reflect that revision
- remove :obj: from docs, since it made confusing cross-refs
Improve Tests for Workflow "Intervals" Experiment

For better consistency with n-grams tests, as part of implementing
LilyPond.
Two nonsense changes...

... just wanted to get these out of the way while I'm trying to merge
LilyPond stuff...
Revise LilyPondIndexer for Multiple Annotation Parts

And the tests.
Adjust WorkflowManager's Part-Combination Handling

- fixes huge bug where not all part combos were included
- updated WorkflowManager's Integration EXPECTED_2 Value
- added new integration test where I did count everything by hand
- makes LilyPond easier
Adjust WorkflowManager's Part-Combination Handling

- fixes huge bug where not all part combos were included
- updated WorkflowManager's Integration EXPECTED_2 Value
- added new integration test where I did count everything by hand
- makes LilyPond easier

(cherry picked from commit 6aa5f1c0e524b5b3280938e403b4f21cd7754084)
(cherry picked from commit a5e1052f2e1bf9770c8a21a0e7779cd14812401c)