Maximum packing fraction in functional definition

Maximum packing fraction `max_eta` can be defined in equation of state but not in functional:

```python 
PcSaftFunctional(PcSaftParameters.from_records([PureRecord(id1, mw1, PcSaftRecord(m, sigma/ANGSTROM, 0.6*epsilon_k/KELVIN)), PureRecord(id2, mw1, PcSaftRecord(m, sigma/ANGSTROM, epsilon_k/KELVIN))], np.array([[0.0, 0.0], [0.0, 0.0]])), max_eta=0.73)
```