[Thermo] add setter TPX to PureFluid

* setter for `PureFluid.TPX` property is added to allow for automatic
restoration of consistent thermodynamic data
* alternative to 'TP', 'TX' or 'PX' which may not uniquely describe
a valid thermodynamic state
* add unit test

interfaces/cython/cantera/test/test_purefluid.py

@@ -149,11 +149,16 @@ class TestPureFluid(utilities.CanteraTest):
 
     def test_TPX(self):
         self.water.TX = 400, 0.8
-        T,P,X = self.water.TPX
+        T, P, X = self.water.TPX
         self.assertNear(T, 400)
         self.assertNear(X, 0.8)
-        with self.assertRaises(AttributeError):
-            self.water.TPX = 500, 101325, 0.3
+
+        self.water.TPX = T, P, X
+        self.assertNear(X, 0.8)
+        with self.assertRaises(ValueError):
+            self.water.TPX = T, .999*P, X
+        with self.assertRaises(ValueError):
+            self.water.TPX = T, 1.001*P, X
 
 
 # To minimize errors when transcribing tabulated data, the input units here are:

interfaces/cython/cantera/thermo.pyx

@@ -1564,9 +1564,24 @@ cdef class PureFluid(ThermoPhase):
             return self.T, self.density, self.X
 
     property TPX:
-        """Get the temperature [K], pressure [Pa], and vapor fraction."""
+        """
+        Get/Set the temperature [K], pressure [Pa], and vapor fraction of a 
+        PureFluid.
+
+        An Exception is raised if the thermodynamic state is not consistent.
+        """
         def __get__(self):
             return self.T, self.P, self.X
+        def __set__(self, values):
+            T = values[0] if values[0] is not None else self.T
+            P = values[1] if values[1] is not None else self.P
+            X = values[2] if values[2] is not None else self.X
+            if np.isclose(P, self.thermo.satPressure(T)):
+                self.TX = T, X
+            elif np.isclose(X, 0.) or np.isclose(X, 1.):
+                self.TP = T, P
+            else:
+                raise ValueError('invalid thermodynamic state')
 
     property UVX:
         """