adds function StkFrames::copyChannel

signature looks like - 
 ```c++
StkFrames& copyChannel(unsigned int sourceChannel,StkFrames& destinationFrames, unsigned int destinationChannel) const;
```
 It copies the specified `sourceChannel` to `destinationFrames`'s `destinationChannel`.  Then returns `destinationFrames` This is useful for extracting individual channels  of an `StkFrames` object and applying  analysis/dsp on them.