Extended PersistenceAgent.as_ly() with candidacy=False keyword.

The method performs as before when candidacy=False.

The method performs differenly when candidacy=True: the method writes output
when no output LilyPond file currently exists; the method also writes output
when an output LilyPond file currently exists and houses different content; but
the method refuses to write output when a comparatively similar LiliyPond file
curently exists.

The intent is to allow composers to set candidacy=True when they want to avoid
writing an essentially duplicative LilyPond output file.