[Positioning] UserInterface: add --verbose option

Currently, --verbose / -v is only a switch. It would probably be better
to have a verbose level instead.

owlps-positioning/src/userinterface.cc

@@ -78,6 +78,7 @@ void UserInterface::fill_cli_options()
   ("config-file,f", po::value<string>()
    ->default_value(DEFAULT_CONFIG_FILE_NAME),
    "Alternative configuration file.")
+  ("verbose,v", "Verbose mode.")
   ; // End of options
 }