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

WHY IS THIS CHANGE NEEDED?:
- grouper (which usues chainer) breaks if it accesses a channel that is
  not GFP

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- chainer assumes all experiments have a GFP channel; this was
  hard-coded in line 29 in chainer.py

WHAT SIDE EFFECTS DOES THIS CHANGE HAVE?:
- yet to test

EVIDENCE THAT COMMIT WORKS:
- pipelines pending

REFERENCES:
- ...