Relates to #52

WHY IS THIS CHANGE NEEDED?:
- IndexError raised when NameGrouper used to define a grouper object
  and then when .concat_signal() was invoked

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- effectively reverted to aliby@252915c6
- the two commits leading up to that one solved the problem: first by
  changing chainer's assumption that all experiments have a GFP channel,
  then updated references to renamed variables

WHAT SIDE EFFECTS DOES THIS CHANGE HAVE?:
- risks: may have broken somewhere upstream not otherwise specified yet

EVIDENCE THAT COMMIT WORKS:
- running grouper works

REFERENCES:
- issue #35

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