Add clearStore call to DblEvSel::next() so it can be called twice ATEAM-918
DoubleEventSelectorAthenaPool::next() stores AttributeList object in StoreGate, so it needs a clearStore() in order to work when called more than once in a row
DoubleEventSelectorAthenaPool::next() stores AttributeList object in StoreGate, so it needs a clearStore() in order to work when called more than once in a row