Fix a few warnings when compiling

[jmcarcell]

BEGINRELEASENOTES

• Fix warnings after Introduce a labels namespace for the string constants EDM4hep#315
• Fix a warning by adding a default case in a switch statement

ENDRELEASENOTES

[tmadlener]

Is this currently warning free? Then we could consider adding -Werror to CI to keep it that way? But I suppose that would need to go into the key4hep build action, right?

[jmcarcell]

I don't get any warnings with this change. Adding -Werror won't work for k4EDM4hep2LcioConv so I can't add it globally yet but I'm not opposed to have it or at least try it for some time. It may be annoying when changing compiler or version and finding that a warning appears though.

[tmadlener]

Maybe it could be an option of the action? Then it would be easier to toggle on a per project basis. Maybe we need to take the discussion to the correct repo. I will merge this now.

[jmcarcell]

Tagging @Zehvogel since he has complained about message verbosity; this change will modify the cases when the Gaudi message level is set to NIL, VERBOSE, FATAL, ALWAYS, NUM_LEVELS (see https://gitlab.cern.ch/gaudi/Gaudi/-/blob/master/GaudiKernel/include/GaudiKernel/IMessageSvc.h#L25), since these were not in the switch statement. At the very least m_level is set by default to NIL: https://gitlab.cern.ch/gaudi/Gaudi/-/blob/master/install/include/GaudiKernel/CommonMessaging.h#L130. I think this may change the default when the verbosity is unset (and defaults to NIL?) since it's ERROR (https://github.com/key4hep/k4MarlinWrapper/blob/main/k4MarlinWrapper/k4MarlinWrapper/MarlinProcessorWrapper.h#L68) and after this it will be MESSAGE.