active_voices indexer documentation

[musicus]

1) There is documentation, but it is not being picked up by sphinx. 2) It is also not clear from the documentation what the goal of the object is.

The documentation should not just show self-documenting code, but should include some type of prose of what it does, and why it is useful.

All code examples in the run() method of the documentation string need to look as follows:

>>> import music21
>>> from vis.analyzers.indexers import noterest, active_voices
>>> score = music21.converter.parse('example.xml')
>>> notes = noterest.NoteRestIndexer(score).run()
>>> av = active_voices.ActiveVoicesIndexer(notes).run()
>>> print(av)

In addition, rather than calling it example.xml perhaps a file from the vis/tests/corpus/ directory should be referenced here.

It would also be great to see some type of expected outcome here, similar to let's say a doctest, although it does not have to be a doctest.

[musicus]

Updated docstring code to be pep8 and Sphinx compliant in develop branch.