Make the event number 64 bits and unsigned

Likely to run into issues with the range of 32 bit integers and the
exepected event numbers. LHC is going beyond 32 bit range already in
long runs.

edm4hep.yaml

@@ -243,7 +243,7 @@ datatypes:
     Description: "Event Header. Additional parameters are assumed to go into the metadata tree."
     Author: "EDM4hep authors"
     Members:
-      - int32_t eventNumber    // event number
+      - uint64_t eventNumber    // event number
       - int32_t runNumber      // run number
       - uint64_t timeStamp     // time stamp
       - double weight       // event weight