Predefined mixtures working in PropsSI now

Added Catch test

``` python
In [1]: import CoolProp

In [2]: CoolProp.CoolProp.PropsSI('D','P',101325,'T',300,'Air.mix')
Out[2]: 1.1766975266680577
```

Closes https://github.com/CoolProp/CoolProp/issues/287

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