Mechanism to set a default sampling rate / maximum amplitude.

[pchanial]

I am thinking about something like that:

import numpy as np
assert Track().rate == 44100
assert Track().max_amplitude == 1.
with default_config(rate=22050, max_amplitude='int16'):
    assert Track().rate == 22050
    assert Track().max_amplitude == np.iinfo(np.int16).max
    assert Track(rate=11025) == 11025
    assert Track(max_amplitude='int32').max_amplitude == np.iinfo(np.int32).max
assert Track().rate == 44100
assert Track().max_amplitude == 1.