DecayTreeFitterAlg requires LHCb::RecVertex

See mattermost starting from https://mattermost.web.cern.ch/lhcb/pl/hiim6cn4bpnyubjbnm4g33piyh by @yamhis

The workaround is

    conf = PersistRecoConf.PersistRecoPacking(stream='/Event/' + stream_pvs, data_type=data_type)
    pvs_v1 = conf.unpackers_by_key()["PVs"]

by @sstahl .

It is decided not to fix for the time being because DecayTreeFit::Fitter needs LHCb::VertexBase which v2::RecVertex does not implement. It will be fixed once the event model is stable.