Minor tweak to allow multiple Indexer about thermosteam HOT 2 CLOSED

    try:
        vol = self._data_cache['vol', TP]
    except:
        chemicals = self.chemicals
        mol = self.data
        vol = np.zeros_like(mol, dtype=object)
        for i, chem in enumerate(chemicals):
            vol[i] = VolumetricFlowProperty(chem.ID, mol, i, chem.V,
                                            TP, None, self._phase)
        self._data_cache['vol', TP] = vol = ChemicalVolumetricFlowIndexer.from_data(property_array(vol),
                                                      self._phase, chemicals,
                                                      False)