THcHodoscope: clear fGoodFlags for each event. Add bugchecks.

fGoodFlags holds event-by-event data, but was never cleared,
eventually growing to several hundred megabytes in a long analysis.
The effciency calculations in THcHodoEff that rely on the contents
of fGoodFlags have been most certainly garbage up to now.

I've replaced the event-by-event ClearEvent() method with the
standard virtual function Clear() and have added fGoodFlags.clear()
there.

Also, I've added assertions to the hodoscope code to check
the validity of some non-obvious array indices. Nothing triggers;
the index logic appears sound (despite its appearance ...)