B2OC: fix bug in basic_builder.py/_make_fourbody: from F.PT to F.SUM(F.PT)
see https://mattermost.web.cern.ch/lhcb/pl/p6f18dhpojg3xbkpw3zfnepg3c from Martin Tat and the few following posts
Edited by Alessandro Bertolin
see https://mattermost.web.cern.ch/lhcb/pl/p6f18dhpojg3xbkpw3zfnepg3c from Martin Tat and the few following posts