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>

include/CoolProp.h

@@ -91,7 +91,7 @@ You might want to start by looking at CoolProp.h
     double saturation_ancillary(const std::string &fluid_name, const std::string &output, int Q, const std::string &input, double value);
 
     /// Get a globally-defined string
-    /// @param ParamName A string, one of "version", "errstring", "warnstring", "gitrevision", "FluidsList", "fluids_list", "parameter_list"
+    /// @param ParamName A string, one of "version", "errstring", "warnstring", "gitrevision", "FluidsList", "fluids_list", "parameter_list","predefined_mixtures"
     /// @returns str The string, or an error message if not valid input
     std::string get_global_param_string(std::string ParamName);
 

src/Backends/Helmholtz/MixtureParameters.cpp

@@ -37,6 +37,16 @@ class PredefinedMixturesLibrary{
 };
 static PredefinedMixturesLibrary predefined_mixtures_library;
 
+std::string get_csv_predefined_mixtures()
+{
+    std::vector<std::string> out;
+    for (std::map< std::string, Dictionary >::iterator it = predefined_mixtures_library.predefined_mixture_map.begin(); it != predefined_mixtures_library.predefined_mixture_map.end(); ++it)
+    {
+        out.push_back(it->first);
+    }
+    return strjoin(out, ",");
+}
+
 bool is_predefined_mixture(const std::string name, Dictionary &dict){
     if (predefined_mixtures_library.predefined_mixture_map.find(name) != predefined_mixtures_library.predefined_mixture_map.end()){
         dict = predefined_mixtures_library.predefined_mixture_map[name];

src/Backends/Helmholtz/MixtureParameters.h

@@ -15,6 +15,11 @@ std::string get_csv_mixture_binary_pairs();
  * 
  */
 bool is_predefined_mixture(const std::string name, Dictionary &dict);
+
+/** \brief Get a comma-separated list of predefined mixtures in 
+ * 
+ */
+std::string get_csv_predefined_mixtures();
     
 /** \brief Get a string for the given binary pair
  * 

src/CoolProp.cpp

@@ -307,6 +307,7 @@ double Props1SI(const std::string &FluidName, const std::string &Output)
 double PropsSI(const std::string &Output, const std::string &Name1, double Prop1, const std::string &Name2, double Prop2, const std::string &Ref)
 {
     std::string backend, fluid;
+	std::vector<double> fractions;
     #if !defined(NO_ERROR_CATCHING)
     // In this function the error catching happens;
     try{
@@ -314,7 +315,12 @@ double PropsSI(const std::string &Output, const std::string &Name1, double Prop1
         // BEGIN OF TRY
         // Here is the real code that is inside the try block
         extract_backend(Ref, backend, fluid);
-        double val = _PropsSI(Output, Name1, Prop1, Name2, Prop2, backend, fluid, std::vector<double>());
+		Dictionary dict;
+		if (is_predefined_mixture(fluid, dict)){
+			fractions = dict.get_double_vector("mole_fractions");
+			fluid = strjoin(dict.get_string_vector("fluids"),"&");
+		}
+        double val = _PropsSI(Output, Name1, Prop1, Name2, Prop2, backend, fluid, fractions);
         if (get_debug_level() > 1){ std::cout << format("_PropsSI will return %g",val) << std::endl; }
         return val;
         // END OF TRY
@@ -498,6 +504,9 @@ std::string get_global_param_string(std::string ParamName)
     }
     else if (!ParamName.compare("parameter_list") ){
         return get_csv_parameter_list();
+    }
+	else if (!ParamName.compare("predefined_mixtures") ){
+		return get_csv_predefined_mixtures();
     }
     else{
         throw ValueError(format("Input value [%s] is invalid",ParamName.c_str()));
@@ -506,8 +515,8 @@ std::string get_global_param_string(std::string ParamName)
 #if defined(ENABLE_CATCH)
 TEST_CASE("Check inputs to get_global_param_string","[get_global_param_string]")
 {
-    const int num_good_inputs = 7;
-    std::string good_inputs[num_good_inputs] = {"version", "gitrevision", "fluids_list", "incompressible_list_pure", "incompressible_list_solution", "mixture_binary_pairs_list","parameter_list"};
+    const int num_good_inputs = 8;
+    std::string good_inputs[num_good_inputs] = {"version", "gitrevision", "fluids_list", "incompressible_list_pure", "incompressible_list_solution", "mixture_binary_pairs_list","parameter_list","predefined_mixtures"};
     std::ostringstream ss3c;
     for (int i = 0; i<num_good_inputs; ++i){
         ss3c << "Test for" << good_inputs[i];

src/Tests/CoolProp-Tests.cpp

@@ -1322,6 +1322,17 @@ TEST_CASE("Test that saturation solvers solve all the way to T = Tc", "[sat_T_to
     }
 }
 
+TEST_CASE("Predefined mixtures", "[predefined_mixtures]")
+{
+	SECTION("PropsSI"){
+		double val = PropsSI("Dmolar","P",101325,"T",300,"Air.mix");
+		std::string err = get_global_param_string("errstring");
+		CAPTURE(val);
+		CAPTURE(err);
+		CHECK(ValidNumber(val));
+	}
+}
+
 TEST_CASE("Test that reference states are correct", "[reference_states]")
 {
     std::vector<std::string> fluids = strsplit(CoolProp::get_global_param_string("fluids_list"),',');