Cleanup & Extend CaloCellID, add 'dense' index
- rename CaloCellID into LHCb::Calo::CellID, and add a 'using' declaration for backwards compatibility
- add LHCb::Calo::Index which, given a CellID, returns a 'dense' sequential index
- adopt code in CaloUtils and CaloKernel to the presence of the LHCb::Calo namespace
- remove unused member functions of CellID
- prefer std::string_view as argument over const std::string&