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

def wants(self, sp_entity_id, index=None):

    return self.metadata.attribute_requirement(sp_entity_id, index)
