Adds an external reference to the role
        @param ext_ref: the external reference object
        @type ext_ref: L{CexternalReference}

def add_external_reference(self,ext_ref):
        """
        Adds an external reference to the role
        @param ext_ref: the external reference object
        @type ext_ref: L{CexternalReference}
        """
        #check if the externalreferences sublayer exist for the role, and create it in case
        node_ext_refs = self.node.find('externalReferences')
        ext_refs = None
        if node_ext_refs == None:
            ext_refs = CexternalReferences()
            self.node.append(ext_refs.get_node())
        else:
            ext_refs = CexternalReferences(node_ext_refs)
        
        ext_refs.add_external_reference(ext_ref)