error reading retrieval from FHIR in ODM example

Hello,

any idea why the following error pops up?

```python
from odmlib import odm_loader, loader

def load_study_odm_v2_0(filepath: str):
engine = loader.ODMLoader(odm_loader.XMLODMLoader(
model_package="odm_2_0", ns_uri="http://www.cdisc.org/ns/odm/v2.0")
)
engine.open_odm_document(filepath)

return engine.load_odm()

load_study_odm_v2_0("example.xml")

---------------------------------------------------------------------------
NameError Traceback (most recent call last)
[](https://localhost:8080/#) in ()
----> 1 load_study_odm_v2_0("example.xml")

10 frames
[](https://localhost:8080/#) in load_study_odm_v2_0(filepath)
1 def load_study_odm_v2_0(filepath: str):
----> 2 engine = loader.ODMLoader(odm_loader.XMLODMLoader(
3 model_package="odm_2_0", ns_uri="http://www.cdisc.org/ns/odm/v2.0")
4 )
5 engine.open_odm_document(filepath)

[/usr/local/lib/python3.10/dist-packages/odmlib/odm_loader.py](https://localhost:8080/#) in __init__(self, model_package, ns_uri, local_model)
75 self.ODM = importlib.import_module(f"{model_package}.model")
76 else:
---> 77 self.ODM = importlib.import_module(f"odmlib.{model_package}.model")
78 self.nsr = NS.NamespaceRegistry()
79

[/usr/lib/python3.10/importlib/__init__.py](https://localhost:8080/#) in import_module(name, package)
124 break
125 level += 1
--> 126 return _bootstrap._gcd_import(name[level:], package, level)
127
128

/usr/lib/python3.10/importlib/_bootstrap.py in _gcd_import(name, package, level)

/usr/lib/python3.10/importlib/_bootstrap.py in _find_and_load(name, import_)

/usr/lib/python3.10/importlib/_bootstrap.py in _find_and_load_unlocked(name, import_)

/usr/lib/python3.10/importlib/_bootstrap.py in _load_unlocked(spec)

/usr/lib/python3.10/importlib/_bootstrap_external.py in exec_module(self, module)

/usr/lib/python3.10/importlib/_bootstrap.py in _call_with_frames_removed(f, *args, **kwds)

[/usr/local/lib/python3.10/dist-packages/odmlib/odm_2_0/model.py](https://localhost:8080/#) in
54
55
---> 56 class StudyEventDef(OE.ODMElement):
57 """ represents ODM v2.0 StudyEventDef and can serialize as JSON or XML """
58 OID = T.OID(required=True)

[/usr/local/lib/python3.10/dist-packages/odmlib/odm_2_0/model.py](https://localhost:8080/#) in StudyEventDef()
62 Category = T.String(required=False)
63 Description = T.ODMObject(element_class=Description)
---> 64 ItemGroupRef = T.ODMListObject(element_class=ItemGroupRef)
65 WorkflowRef = T.ODMListObject(element_class=WorkflowRef)
66 Alias = T.ODMListObject(element_class=Alias)

NameError: name 'ItemGroupRef' is not defined
```

where `example.xml` is the provided [example for retrieval from FHIR in ODM v2.0](https://github.com/cdisc-org/DataExchange-ODM/blob/main/examples/FHIR_Integration/Data_Retrieval_From_FHIR_in_ODM.xml).