[ModelicaSystem] add type hint for return value of isParameterChangeable()

Author: syntron

OMPython/ModelicaSystem.py

@@ -1178,7 +1178,7 @@ def _set_method_helper(
     def isParameterChangeable(
             self,
             name: str,
-    ):
+    ) -> bool:
         q = self.getQuantities(name)
         if q[0]["changeable"] == "false":
             return False