Add ProcessSampleBuffer functions to various audio effects

[JimBayne]

This update provides 2 new functions for most audio effects: 1) public bool ProcessSampleBuffer(in IntPtr sampleBuffer, in int Channel, in int nChannels, in int frameCount)

• allows unmanaged audio buffers of (possibly) interleaved channels to be processed with a single call per channel
• can be used to directly process audio buffers presented by various audio api's such as PortAudio 2) public bool ProcessSampleBuffer(in float[] sampleBuffer, in int Channel, in int nChannels, in int frameCount)
• allows managed audio buffers of (possibly) interleaved channels to be processed with a single call per channel

The intent is to improve performance and reduce overhead by reducing the # of calls, and performing the updates directly in the source audio buffers.

I use the IntPtr versions in my GuitarAmigo app. They perform very well.

[ar1st0crat]

I suppose there's nothing wrong with this PR. But the philosophy of this lib is more about readability than performance optimization. I didn't want to mix C# and C code from the very beginning, although I realize C parts would run faster (however, I'm not sure that the difference will be noticeable, especially nowadays). Also, there is already the Process(buffer) method implemented for all online filters (including audio effects). And all corresponding current implementations have satisfactory performance.