Adds a new property to the property layer
        @type pid: string
        @param pid: property identifier
        @type label: string
        @param label: the label of the property
        @type term_span: list
        @param term_span: list of term identifiers

def add_property(self,pid, label,term_span):
        """
        Adds a new property to the property layer
        @type pid: string
        @param pid: property identifier
        @type label: string
        @param label: the label of the property
        @type term_span: list
        @param term_span: list of term identifiers
        """
        node_prop = self.node.find('properties')
        if node_prop is None:
            properties = Cproperties(type=self.type)
            self.node.append(properties.get_node())
        else:
            properties = Cproperties(node=node_prop,type=self.type)
            
        properties.add_property(pid, label,term_span)