Changes to CaloClusters v2

Changes required for Rec!2080 (merged)

• Add center (used to store the center-of-gravity of a cluster) to v2 Cluster for backwards compatibility]
• More 'transparant' proxy for clusters, in order to be able to set eg. center for the non-cost case, but unable to do so for the const case.
• Tweak proxy to avoid signed-unsigned conversions
• Add 'reserve' for both old hypos and relations2d to speed up converters from new->old hypos, clusters, tables
• point 'packers' to converted hypo location
• Remove 'split photons' from persistency

must be applied in conjunction with Rec!2080 (merged) (and the other MR that go with it!)