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?:
- if a signal is not present in all HDFs files in a directory, defining
  a grouper object and then invoking
  grouper.concat_signal('<name_of_signal>') throws an error

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- modify pool_function() so that a dictionary of signals is an argument.
  this dictionary of signals indicates which files have the signal of interest.
- pass the dictionary of signals into concat_signal(), so that it knows
  which ones to process (i.e. concatenate)