Added REFPROP_lib.h from 4.x

Signed-off-by: Ian bell <ian.h.bell@gmail.com>
CoolProp v5 now compiles with the REFPROP backend.  Core code working.

Signed-off-by: Ian bell <ian.h.bell@gmail.com>
Merge branch 'v5' of https://github.com/coolprop/coolprop into v5