History: Scalers for GEn

First I'll punt and point to Bob Michael's general page on the THaScaler class.

What is in the GEN tree (and TTree variables "gen_" that are just copies)
is actually defined by the THaScaler class. The different
"trigger" types are pretty clear, however be aware that by default
everything except dclock is stored in the TTree as a "RATE". The clock variable is just the counting of the actual clock signal, divided by the time (in seconds) so that gives you the 105kHz frequency. The dclock variable I recently added to the class is the number of clock-cycles that the scaler event covers, and is present so we can get back from the rates to actual trigger yields and accumulated charges.