This test is used to make sure that the basic functions in
the water property routine satisfies the derivative tests
specified in the paper:

W. Wagner, A. Prub, "The IAPWS Formulation 1995 for the
Thermodynamic Properties of Ordinary Water Substance
for General and Scientific Use," J. Phys. Chem.
Ref. Data, v. 31, 387 - 442, 2002.

On page 436 of that reference, value for phi, the nondimensional
helmholtz free energy and its first and second derivatives,
are given at 2 representative points. This test makes sure
that the values agree with the paper's values.