Database use now mode instead of create to allow more versatile bheaviour

navipy/database/__init__.py

@@ -61,6 +61,7 @@ class DataBase():
               channels: list
         """
         self._logger = logging.getLogger('navipy')
+        self.__convention = None
         if not isinstance(filename, str):
             msg = 'filename should be a string'
             self._logger.exception(msg)