Add per event purge of ShowerClusterList in order to reduce grow of
memory consumption with event number.

       Add THcShowerClusterList::purge() method to purge and clean
       THcShower::fClusterList by the end of each event, in
       THcShower::Clear().
Move energy to track assignment from THcHallCSpectrometer to the
shower counter's FineProcess.

       Remove THcHallCSpectrometer::TrackEnergies method.
       Add GetShEnergy call for each tack in THcShower::fineProcess.