fix(extraction): define candidate channels

WHY IS THIS CHANGE NEEDED?:
- invoked grouper (which used chainer, which in turn is based on
  Signal), encountered AttributeError: 'Chainer' object has no
  attributes 'candidate_channels'

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- chainer used to rely on av_channels in
  extraction/core/functions/defaults.py, so renamed that to
  candidate_channels
- added a candidate_channels attribute to Signal object to fulfil references