[ModelicaSystem] improve handling of variable filter in buildModel()

* do NOT overwrite class definition

Author: syntron

OMPython/ModelicaSystem.py

@@ -469,15 +469,18 @@ def getWorkDirectory(self) -> pathlib.Path:
         return self._tempdir
 
     def buildModel(self, variableFilter: Optional[str] = None):
+        filter_def: Optional[str] = None
         if variableFilter is not None:
-            self._variable_filter = variableFilter
+            filter_def = variableFilter
+        elif self._variable_filter is not None:
+            filter_def = self._variable_filter
 
-        if self._variable_filter is not None:
-            varFilter = f'variableFilter="{self._variable_filter}"'
+        if filter_def is not None:
+            var_filter = f'variableFilter="{filter_def}"'
         else:
-            varFilter = 'variableFilter=".*"'
+            var_filter = 'variableFilter=".*"'
 
-        buildModelResult = self._requestApi("buildModel", self._model_name, properties=varFilter)
+        buildModelResult = self._requestApi("buildModel", self._model_name, properties=var_filter)
         logger.debug("OM model build result: %s", buildModelResult)
 
         self._xml_file = pathlib.Path(buildModelResult[0]).parent / buildModelResult[1]