In this example, the moles of the state are now correctly obtained in a density iteration instead of default values.

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()
>>> 100 kPa

closes #25

To facilitate testing, this PR also adds a Python constructor for PengRobinsonRecord.