Add differential expression information to the attribute mapping dictionary.

        :param int att_ind_start: Start index for enumerating the attributes.
        :param dict att_mappings: Dictionary of mappings between vertices and enumerated attributes.
        :return: End index for attribute enumeration.

def _add_differential_expression_attributes(self, att_ind_start, att_mappings):
        """Add differential expression information to the attribute mapping dictionary.

        :param int att_ind_start: Start index for enumerating the attributes.
        :param dict att_mappings: Dictionary of mappings between vertices and enumerated attributes.
        :return: End index for attribute enumeration.
        """
        up_regulated_ind = self.graph.vs.select(up_regulated_eq=True).indices
        down_regulated_ind = self.graph.vs.select(down_regulated_eq=True).indices
        rest_ind = self.graph.vs.select(diff_expressed_eq=False).indices

        self._add_attribute_values(att_ind_start + 1, att_mappings, up_regulated_ind)
        self._add_attribute_values(att_ind_start + 2, att_mappings, down_regulated_ind)
        self._add_attribute_values(att_ind_start + 3, att_mappings, rest_ind)
        return att_ind_start + 4