updated GroupRef to new system

src/fiber_package/groups/communication.py

@@ -60,7 +60,7 @@ class ActivityModel(CommunicationModel):
         Field(pattern=CommunicationCategory.activity)
     ]
 
-    signuplist: GroupRef = GroupRef("signuplist", None)
+    signuplist: GroupRef | None = None
     # TODO: validate that a signuplist can only be attached if category is
     # activity? Or should this not be validated?
 

src/fiber_package/groups/signuplist.py

@@ -7,6 +7,7 @@ from fiber_package.util import GroupRef
 A sign-up list is a list of members that have signed up for a given activity.
 '''
 class SignuplistModel(BaseModel):
-    activity: GroupRef = GroupRef("communication", None)
+    activity: GroupRef | None = None
     # TODO: validate that the communication is indeed an activity (or should
     # this be done?)
+    # Should None be allowed?