fix(postproc): group signals apart from GFP

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 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 (closed)