DaVinci analysis tutorial should use PersistReco unpackers

The HLT2 output analysis tutorial does not configure the PersistReco unpackers. Existing lines probably just 'happen' to work because UnpackParticlesAndVertices might try to unpack some reconstruction objects.

We should update the tutorial to explicitly use the unpackers. It could take advantage of the helpers in GaudiConf.reading, which are being introduced in LHCb!2927 (merged).

/cc @nskidmor