'Returns what attributes the SP requires and which are optional
if any such demands are registered in the Metadata.
:param entity_id: The entity id of the SP
:param index: which of the attribute consumer services its all about
if index=None then return all attributes expected by all
attribute_consuming_services.
:return: 2-tuple, list of required and list of optional attributes'

def attribute_requirement(self, entity_id, index=None):

    raise NotImplementedError
