Fix: issue #98. Add impl of required abstract method

flamapy/metamodels/fm_metamodel/operations/fm_metrics.py

@@ -39,6 +39,9 @@ def __init__(self) -> None:
         self._constraints_per_features: list[int] = []
         self._feature_ancestors: list[int] = []
 
+    def model_type_extension(self) -> str:
+        return 'fm'
+
     def get_result(self) -> list[dict[str, Any]]:
         return self.result