[FR]: Direct (Juce audio buffer like) access to channels in the input and output buffer

[leanmeanaudio]

Is your feature request related to a problem? Please describe.
People are asking to send certain channels to multiple outputs. If we can just copy channels on the output buffer, adding this would be super easy.

Describe the solution you'd like
Some kind of API to have juce audiobuffer like access at the start and at the end of processing.

Describe alternatives you've considered
Set multiple output tracks of 1 track could be a "simpler" but less flexible solution.