Extract signals needed for Helicity decoding.

  These signals are plus and minus signals, the MPS indicator, and
  the start of quartet indicator.  The time from the TI (with overflow
  detection) is also retrieved as this is needed when dealing with
  delayed helicity reporting.

  The helicity signals are in FADC250 channels.  The roc/slot/channel
  assignments for these signals are currently hardwired into the code.