Draft: Add an algorithm to get the current valid DeMagnet from the TES

As discussed this morning at the Core Software Meeting, the easiest way to pass condition data to tools (when using bound tools is not practical) is to add a reference to the condition objects in the TES. So the new algorithm LHCb::DetCompat::MagnetStateForEvent adds a LHCb::DetCompat::Magnet (thin wrapper around DeMagnet) instance on the TES, so that tools can be quickly migrated to DeMagnet (instead of MagneticFieldSvc).