State construction branches incorrectly for unorthodoxly specified states

In this example, the state is calculated using N=1 and the given volume instead of calculating the moles from a density iteration.
```python
T = 25*CELSIUS
V = (100*ANGSTROM)**3
p = BAR
z = 3e-4
State(func, T, pressure=p, molefracs=np.array([1-z,z]), volume=V).pressure()
>>> 3.9624kPa
```