implemet local channel mono<->stereo conversion and improve stereo VUs accuracy

* now allows multiple channels per input source
* added LinJam::GetNumInputChannels() method
* added LinJam::AddDecibels() , ComputePannedVus() , ScalePannedMonoVus() methods
* removed LinJam::GetNumActiveLocalChannels() method
* removed LinJam::GetMonoPairSourceN() and GetStereoPairSourceN() methods