Wrap SpreadEstimator in a namespace to avoid clash with the copy in LHCb
The presence of two classes with the same name was causing a minor memory corruption visible only when enabling both gaudi/Gaudi!577 (merged) and LHCb!2663 (merged).
Adding a namespace around the copy in Rec prevents the clash and so the memory corruption.
It's not clear if the copy in LHCb (Calo/CaloUtils) is actually needed.