Open points for PV <-> particle association (Moore MR #2658)

For discussing how to handle different PV associations offline

The following discussion from !3665 (merged) should be addressed:

• @ldufour started a discussion: (+34 comments)

  This will copy the particles to a new keyedcontainer... are we sure the relation tables of the old particles still work on these new ones?

• An LHCbIntegration test needs to be provided showing that the persistency indeed works across the chain LHCbIntegrationTests!61 (merged)

• A way to access the HLT2 PV and the unbiased PV in the same nTuple needs to be provided

• A way to access the unbiased PV information of the daughters

• A documentation of the particle unbiasing in DaVinci needs to be added

• The 'mass constraint' in the vertex fit needs a test, this is currently untested code

• The PV functors need to be able to take explicit relations, as we want to be able to use other PV associations offline (and mix them)