moved mixer3D, myWorld from CAU to Sonic
Finished restructuring, but with linker errors

Currently CustomAudioUnit.cpp includes Sonic.h, in order to have access to
Sonic::onCallback(). This seems kind of hacky though and may be the reason
for the aforementioned linker errors. A better structure would be to rename
CustomAudioUnit iOS_Sonic, then to store the core Sonic library entirely
within this class. iOS_Sonic would have simple wrapper functions for each
one of the core Sonic's public methods, but would implement play() and stop()
in accordance with the iOS requirments (like CustomAudioUnit is doing now).

Essentially the only purpose the core Sonic library is serving is at each
time step to fill the output buffers with the appropriate audio data.
The platform-specific wrapper (in this case iOS_Sonic) is responsible for
then taking that audio and passing it to the hardware.