Adds an external reference for the given markable
        @type markable_id: string
        @param markable_id: the markable identifier
        @type external_ref: L{CexternalReference}
        @param external_ref: the external reference object

def add_external_reference(self,markable_id, external_ref):
        """
        Adds an external reference for the given markable
        @type markable_id: string
        @param markable_id: the markable identifier
        @type external_ref: L{CexternalReference}
        @param external_ref: the external reference object
        """
        if markable_id in self.idx:
            markable_obj = Cterm(self.idx[markable_id],self.type)
            markable_obj.add_external_reference(external_ref)
        else:
            print('{markable_id} not in self.idx'.format(**locals()))