Phase Envelope calculations now include Tsat_max and psat_max, along with indices thereof.

The Tsat_max and psat_max values are calculated using exact solution based on finding dT/dP and dp/dT equaling zero by a 1D solution in rhov.

See also https://github.com/CoolProp/CoolProp/issues/133

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>