Adds the objective(s) to the sbml document.

def _add_fbc_objective(self, model_tag, obj_id):
        """Adds the objective(s) to the sbml document."""
        objective_list = ET.SubElement(model_tag, _tag(
            'listOfObjectives', FBC_V2))
        objective_list.set(_tag('activeObjective', FBC_V2), 'O_1')
        objective_tag = ET.SubElement(
            objective_list, _tag('objective', FBC_V2))
        objective_tag.set(_tag('id', FBC_V2), 'O_1')
        objective_tag.set(_tag('type', FBC_V2), 'maximize')
        flux_objective_list = ET.SubElement(objective_tag, _tag(
            'listOfFluxObjectives', FBC_V2))
        flux_objective_tag = ET.SubElement(flux_objective_list, _tag(
            'fluxObjective', FBC_V2))
        flux_objective_tag.set(_tag('reaction', FBC_V2), 'R_' + obj_id)
        flux_objective_tag.set(_tag('coefficient', FBC_V2), '1')