Adds a property to the property layer
        @type label: string
        @param label: the type of property
        @type term_span: list
        @param term_span: list of term ids
        @type pid: string
        @param pid: the identifier for the property (use None to automatically generate one)

def add_property(self,label,term_span,pid=None):
        """
        Adds a property to the property layer
        @type label: string
        @param label: the type of property
        @type term_span: list
        @param term_span: list of term ids
        @type pid: string
        @param pid: the identifier for the property (use None to automatically generate one)
        """
        if self.features_layer is None:
            self.features_layer = Cfeatures(type=self.type)
            self.root.append(self.features_layer.get_node())
        self.features_layer.add_property(pid, label,term_span)