[Rgc_analysis] Cut for FC asymmetry measurement

[tforest at jlab.org]

Fellow collaborators:

Base on today's discussion, I would suggest changing the line in
FCcup_reading.cc from

if(HEL_SCALER.getRows()>0)

to be

 if(HEL_SCALER.getRows()>0 && HEL_SCALER.getFloat("clock",0) > 30000)

It should help ensure reliable results.  Only one run, 16535, had a
relative changed of more than 1% with the suggested cut above.

Tony