Fixed doxygen comments
Added missing value_ member data
First working interface
Added appropriate index (RunIndex, LuminosityBlockIndex or StreamID) needed from the threaded framework.
Conform to new Principal types constructors which require passing in the appropriate index instance (RunIndex, LuminosityBlockIndex or StreamID). The index is needed for the threaded framework.
Allowed default value for StreamID in constructor in order to match the signature of the previous API.

This avoids having to deal with porting changes to the EventPrincipal constructor calls outside of the framework (there are 2 such cases). When we move to the threaded processing those cases will have to be modified anyway to account for multiple Events running simultaneously.
Provide access to the indexes (RunIndex, LuminosityBlockIndex or StreamID).

These indexes are needed to access per transition data in the threaded framework.