Allow monkey patching of relationships on existing classes.  

Allow monkeypatching existing device relationships.

also support enums.
RELATIONSHIPS_YUML = """
[Products.ZenModel.Device.Device]++ -[SystemBoard]
"""

        'ExampleClass': {
            'properties': {
                'location':  {'label': 'Location',
                              'enum': {'close', 'far', 'up','down'},
                             },     
              }
        },

This seems awfully expensive to make mandatory on every ZenPack using zenpacklib regardless of if relations need to be rebuilt or not.