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)