Improve Oversampling for PT8211

See conversation https://github.com/PaulStoffregen/Audio/pull/194

Combine two loops in stereo CIC computation into one for-loop.
Add compiler optimization "unroll-loops" to isr if OS enabled. Speed
improved from 113µs to 65µs per isr call for CIC interpolation filtering
(Measured on Teensy 3.2 96MHz optimized).

NoteFrequency fixes

Add shelf filter support for biquad, add experimental support for PT8211

PT8211 with oversampling and linear interpolation by Frank Bösing.
3 Stage CIC interpolation by Ben Rheinland

fix problem with left+right channels being swapped

AudioConnection:Fix the source and destination names for AudioConnect…

Added a drum synthesis object to Audio library

Removed debug code, updated comments

Adding CPP/H implementation files, plus example sketch and update to
HTML file.