Implements rhythmic-music-iterator (issue 5554048)

[dak]

http://codereview.appspot.com/5554048/diff/2001/lily/rhythmic-music-iterator.cc
File lily/rhythmic-music-iterator.cc (right):

http://codereview.appspot.com/5554048/diff/2001/lily/rhythmic-music-iterator.cc#newcode41
lily/rhythmic-music-iterator.cc:41: report_event (get_music ());
I suppose that one has to clear "articulations" here (after fetching
them), or they will get treated by both report_event as well as the
rhythmic iterator.

Since it would be awkward to actually mess with the Music event, one
will likely not be able to call report_event, but instead call to_event,
and then take the articulations from there.  As a result, one would not
be calling report_event in the loop either.  And one would likely have
to descend_to_context (and other report_event stuff) manually.

http://codereview.appspot.com/5554048/