Added example helper functions to TaleaRhythmMaker.

This message appears as a comment in the TaleaRhythmMaker classfile:

    '''Example helpers:

    # used in a piece with four voices:
    # voice 1 starts reading talea at beginning of talea;
    # voice 2 starts reading talea at second event of talea;
    # voice 3 starts reading talea at third event of talea;
    # voice 4 starts reading talea at fourth event of talea.
    def helper(talea, seeds):
        assert len(seeds) == 2
        if not talea:
            return talea
        voice_index, measure_index = seeds
        talea = sequencetools.rotate_sequence(talea, -voice_index)
        return talea

    # used in a piece with four voices:
    # voice 1 starts reading talea at beginning of talea;
    # voice 2 starts reading talea 1/4 of way through talea;
    # voice 3 starts reading talea 2/4 of way through talea;
    # voice 4 starts reading talea 3/4 of way through talea.
    def quarter_rotation_helper(talea, seeds):
        assert len(seeds) == 2
        if not talea:
            return talea
        voice_index, measure_index = seeds
        index_of_rotation = -voice_index * (len(talea) // 4)
        index_of_rotation += -4 * measure_index
        talea = sequencetools.rotate_sequence(talea, index_of_rotation)
        return talea
    '''

Couple of things to note:

1. the functions are short.

2. the purpose of the functions is to allow four different voices in the piece
to read from four different starting points of the same talea.

What does this mean?

This sort of thinking was the root of what lead to the development of the
StatalServer and StatalServerCursor in datastructuretools.

So two things are possible:

1. rhythm-maker helpers should be deprecated in favor of statal servers.

2. rhythm-maker helpers should be used together with statal servers.

It's not immediately clear which path is the best one.