Implement Gordon-Smith 2-pole oscillator

+ ensure that new audio path is completely out of the way when it is not enabled (e.g., extra buffer copies)
AFAICT - this code doesn't get used. And it even appears as if the AudioSourceTone never compiles (since nowhere in the code do we even instantiate an instance of it). Did you intended this to be the case?

Also, there's really far too much "business logic" in this header file. The implementations of these functions needs to be broken out into a .cpp file. 

> **2.1.3. Most function implementations should reside in the source file.**
> The header files should declare an interface, the source file should implement 
> it. When looking for an implementation, the programmer should always know 
> that it is found in the source file.