add explicit attribute self. class_matches

add explicit attribute `self.cless_matches` to make class `NPLinker` workable.

src/nplinker/loader.py

@@ -132,6 +132,7 @@ def __init__(self, config_data):
         self.webapp_scoring_cutoff = self._config_webapp.get(
             "tables_metcalf_threshold", self.TABLES_CUTOFF_DEFAULT
         )
+        self.class_matches = None
         self.chem_classes = None
 
         logger.debug(