Migrate CaloSinglePhotonAlg to ScalarTransformer
NOTE: Rebased on top of !169 (merged) -- please make sure !169 (merged) is merged first, so this will be a single-commit instead of fifty commits... (and then delete this NOTE)
Use the scalar-to-vector adaptor of ScalarTransformer to do the loop over CaloClusters to create CaloHypos, only provide the scalar transform of CaloCluster to (optional) CaloHypo explicitly.