Undoes delayed helicity reporting by learning seed of pseudo random
sequence used by injector helicity electronics.

This detector class should be added to the Trigger apparatus.
Delivers the global variables "hel" and "helrep".  "helrep" is the
reported delayed helicity and "hel" is the actual helicity.  + and -
helicity are reported as +1 and -1.  If the event is during a MPS
settling period, then the helicity variables are zero.  For the first
few seconds of a run, the "hel" variable will be zero as the pseudo
random sequence seed has not been determined.  (If the trigger class
is "T" and this detector is called "helicity", then these variables
will be T.helicity.hel and T.helicity.helrep.

By default, it is assumed that there is delayed reporting of 8 cycles
and that Quartets are used.  If delayed helicity reporting is not in
use, then set the parameter "helicity_delay" to zero.