Added segment-maker method to get existing music-maker.

Example:

    >>> segment_maker.get_music_maker(voice_name='Cello Voice', stage=2)

The examples asks the segment-maker to iterate its list of music-makers. During
iteration the segment-maker looks for any music-maker with voice name equal to
'Cello Voice' and which is defined for stage 2.

Provides a semantic way of retrieving previously defined music-makers.