Larger MCRecoAssocation collection than RecoParticles collection when using DelphesPythia8_EDM4HEP

Migrated issue: https://github.com/key4hep/EDM4hep/issues/91

QUOTE:
In the process of implementing reco to truth matching in FCCAnalyses, I realised that the fact that we see a larger number of MCRecoAssocation collection than RecoParticles collection is mainly due to a generated pi0 decaying to two photons which are reconstructed as a single photon. I did not made a super detailed study, but I'm pretty sure this could happened with other types of particles. In such cases we have the same reco particle pointing to two MC particles in two different OneToOneRelations. Won't it be better to have OneToManyRelations in such cases?

Below is an interesting event at FCC-hh, very busy and rare one as it is ttHH.
In this event we have 6324 MC particles, 1224 reconstructed particles and 1234 associations (OneToOneRelation MC reco)
You can see that many of the reconstructed particles have in the event even build with up to 4 truth particles, from the same or different parents. I know we are restructuring the code, but I wanted to share this here for the record.
```
rec ind 549 reco P 24.269 mc ind 5528 truth P 0.859872 pdg_id 22 parent id 4456 parent pdg id 111
rec ind 549 reco P 24.269 mc ind 6267 truth P 10.1102 pdg_id 22 parent id 6203 parent pdg id 111
rec ind 549 reco P 24.269 mc ind 6267 truth P 10.1102 pdg_id 22 parent id 6203 parent pdg id 111
rec ind 549 reco P 24.269 mc ind 6288 truth P 13.2324 pdg_id 22 parent id 6230 parent pdg id 111
rec ind 687 reco P 18.5327 mc ind 6176 truth P 9.0619 pdg_id 22 parent id 5836 parent pdg id 111
rec ind 687 reco P 18.5327 mc ind 6177 truth P 9.10258 pdg_id 22 parent id 5837 parent pdg id 111
rec ind 852 reco P 14.8237 mc ind 5344 truth P 8.51735 pdg_id 22 parent id 4125 parent pdg id 111
rec ind 852 reco P 14.8237 mc ind 5379 truth P 5.31896 pdg_id 22 parent id 4251 parent pdg id 221
rec ind 993 reco P 33.1935 mc ind 4318 truth P 21.0651 pdg_id -211 parent id 4252 parent pdg id 21
rec ind 993 reco P 33.1935 mc ind 4721 truth P 13.2629 pdg_id 211 parent id 3982 parent pdg id -313
rec ind 1046 reco P 3.92714 mc ind 3785 truth P 3.28534 pdg_id 211 parent id 3768 parent pdg id 2
rec ind 1046 reco P 3.92714 mc ind 5149 truth P 1.80995 pdg_id 211 parent id 3646 parent pdg id 223
rec ind 1075 reco P 12.9256 mc ind 5323 truth P 2.71686 pdg_id -211 parent id 4054 parent pdg id 223
rec ind 1075 reco P 12.9256 mc ind 5838 truth P 9.04003 pdg_id 211 parent id 4984 parent pdg id 310
rec ind 1144 reco P 18.772 mc ind 3411 truth P 0.607773 pdg_id 211 parent id 3395 parent pdg id 1
rec ind 1144 reco P 18.772 mc ind 3957 truth P 19.3562 pdg_id 2112 parent id 3931 parent pdg id 5
rec ind 1201 reco P 190.908 mc ind 4424 truth P 41.8603 pdg_id 211 parent id 3226 parent pdg id 2
rec ind 1201 reco P 190.908 mc ind 4715 truth P 123.7 pdg_id -321 parent id 3976 parent pdg id -313
rec ind 1213 reco P 38.7616 mc ind 4363 truth P 99.3652 pdg_id -211 parent id 3226 parent pdg id 2
rec ind 1213 reco P 38.7616 mc ind 4549 truth P 33.114 pdg_id -211 parent id 3563 parent pdg id 113
```
ENDQUOTE

Start with the migrated EDM4hep issue 91 and the DelphesPythia8_EDM4HEP association output described here. Review how MCRecoAssocation and RecoParticles are represented, focusing on cases where one reconstructed particle points to multiple MC particles. Done requires an agreed relation model and verification against the supplied event examples.

Written by the indexing model from the issue text.