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

OLD:

    >>> old_music_maker = segment_maker.get_music_maker('Cello Voice', 2)
    >>> new_music_maker = new(
    ...     old_music_maker,
    ...     rhythm_maker__tuplet_ratios=[(1, 4)],
    ...     voice_name='Viola Voice',
    ...     )
    >>> segment_maker.music_makers.append(new_music_maker)

NEW:

    >>> segment_maker.copy_music_maker(
    ...     'Cello Voice',
    ...     2,
    ...     rhythm_maker__tuplet_ratios=[(1, 4)],
    ...     voice_name='Viola Voice',
    ...     )

SegmentMaker.copy_music_maker() is just a short-cut that allows for retrieval
and templating in a single call.

The method makes segment definition.py even more readable.