Add T_freeze as an output for incompressibles

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

src/Backends/Incompressible/IncompressibleBackend.h

@@ -122,6 +122,8 @@ public:
     
     long double calc_fraction_min(void){return fluid->getxmin();};
     long double calc_fraction_max(void){return fluid->getxmax();};
+    long double calc_T_freeze(void){
+        return fluid->Tfreeze(_p, _fractions[0]);};
 };
 
 } /* namespace CoolProp */