docs: 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)