model.ReferenceElement.value: change value_type from AASReference to Reference...

model.ReferenceElement.value: change value_type from AASReference to Reference according the official specification fix #83 (closed)

Todo (after !57 (merged) is merged): Change _construct_referable_reference into _construct_reference in _construct_reference_element: value = _failsafe_construct(element.find(NS_AAS + "value"), _construct_reference, failsafe)