Add is_pure function

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

src/Backends/Helmholtz/HelmholtzEOSMixtureBackend.h

@@ -54,7 +54,7 @@ public:
     bool using_mole_fractions(){return true;}
     bool using_mass_fractions(){return false;}
     bool using_volu_fractions(){return false;}
-    
+    bool is_pure(){ return is_pure_or_pseudopure; }
     bool has_melting_line(){ return is_pure_or_pseudopure && components[0]->ancillaries.melting_line.enabled();};
     long double calc_melting_line(int param, int given, long double value);
     phases calc_phase(void){return _phase;};