Added the ability to use pre-defined mixtures in REFPROP like REFPROP-MIX:R410A.mix

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
CoolPropStateClassSI now supports REFPROP predefined mixtures

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

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
State class can now accept mixtures from REFPROP

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