extend CaloClusters v2 functionality

• make LHCb::CaloDataFunctor::CellID work on any type of object which has either a 'seed()' or 'cellID()' member function and which returns a CaloCellID (needed to make CaloCluster v2 work with the transverse energy functor)

• Allow v2 CaloClusters to be modified (only in case the container is non-const)

  • add a 'sort' member
  • add a (temporary!) BackwardsCompatibilityAdapter which allows to pass v2 clusters to old-style cluster interface, and be updated.
    • this works by making a temporary old-style cluster, from the V2 cluster, passing a reference to old-style one around to be modified, and then finally update the new-style cluster from the old one
    • note that the proper solution is to update tools to accept an entire cluster v2 container, and avoid passing one cluster at the time

Needed for Lbcom!473 (merged)