model.Extension: Change refers_to to be of type List[Reference]

Currently, `Extension.refers_to` is declared as a `Iterable[Reference]`.
This implies, that we can not necessarily check, whether or not the
attribute is empty or not.
This creates a problem with the XML serialization, since the
`<aas.refersTo>` element should only appear if there is at least
one `Reference` inside.

This commit changes the `Extension.refers_to` to be a list of
`Reference`s, as well as adapting a more clear check whether
or not the attribute is empty in `adapter.xml.xml_serialization`.

basyx/aas/adapter/xml/xml_serialization.py

@@ -277,7 +277,7 @@ def extension_to_xml(obj: model.Extension, tag: str = NS_AAS+"extension") -> etr
                                               text=model.datatypes.XSD_TYPE_NAMES[obj.value_type]))
     if obj.value:
         et_extension.append(_value_to_xml(obj.value, obj.value_type))  # type: ignore # (value_type could be None)
-    if obj.refers_to:
+    if len(obj.refers_to) > 0:
         refers_to = _generate_element(NS_AAS+"refersTo")
         for reference in obj.refers_to:
             refers_to.append(reference_to_xml(reference, NS_AAS+"reference"))

basyx/aas/model/base.py

@@ -1458,7 +1458,7 @@ def __init__(self,
                  name: NameType,
                  value_type: Optional[DataTypeDefXsd] = None,
                  value: Optional[ValueDataType] = None,
-                 refers_to: Iterable[ModelReference] = (),
+                 refers_to: List[ModelReference] = (),
                  semantic_id: Optional[Reference] = None,
                  supplemental_semantic_id: Iterable[Reference] = ()):
         super().__init__()
@@ -1468,7 +1468,7 @@ def __init__(self,
         self.value_type: Optional[DataTypeDefXsd] = value_type
         self._value: Optional[ValueDataType]
         self.value = value
-        self.refers_to: Iterable[ModelReference] = refers_to
+        self.refers_to: List[ModelReference] = refers_to
         self.semantic_id: Optional[Reference] = semantic_id
         self.supplemental_semantic_id: ConstrainedList[Reference] = ConstrainedList(supplemental_semantic_id)