Add 'AudioClip.max_volume(stereo=True)' support for more than 2 channels

[mondeja]

Next snippet of code demonstrates the bug:

make_frame = lambda t: np.array([
    np.sin(t * 0),
    np.sin(440 * 2 * np.pi * t),
    np.sin(t * 0),
    np.sin(440 * 2 * np.pi * t),
]).T.copy(order="C")

clip = AudioClip(make_frame, fps=44100, duration=1)
clip.preview()
print(clip.nchannels, clip.max_volume(stereo=True))

The clip has 4 channels but clip.max_volume(stereo=True) returns maximum volume for two first. After this pull returns all of them.

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it
• I have formatted my code using black -t py36

[coveralls]

Coverage remained the same at 66.04% when pulling e8a994b on mondeja:audio-stereo-max-volume into 7c3dc0a on Zulko:master.