Calculate psychrometric properties of moist air.

MOTIVATION
I have been working on this off and on for a few years.  I am offering it now
in the hope it might be useful for others.


REFFERENCES
"Psychrometrics - Theory and Practice" ASHRAE 1996

O. Parish and Terrill Putnam, "Equations for the Dertrmination of Humidy
from the Dewpoint and Psychrometric Data" NASA Tech Note 

"Humidity Conversion Formulas", Vaisala, 2013


STATUS AND USABILITY
Definately pre-alpha
All the PsyState functions that use P have been implemented. They have been 
checked and generally give good results; however in some cases invalid combinations 
are possible. 

Some range checking is in place but it is incomplete.

Functions that calculate P have been spot checked, so they work but the  
P calculated may not be very accurate.