[Thermo] Add compatibility check for phases added to MultiPhase

PureFluidPhase and WaterSSTP are not compatible with MultiPhase since they are
already multi-phase mixtures.

Resolves #306

include/cantera/thermo/PureFluidPhase.h

@@ -161,6 +161,10 @@ public:
     virtual std::string report(bool show_thermo=true,
                                doublereal threshold=1e-14) const;
 
+    virtual bool compatibleWithMultiPhase() const {
+        return false;
+    }
+
 protected:
     //! Main call to the tpx level to set the state of the system
     /*!

include/cantera/thermo/ThermoPhase.h

@@ -1290,6 +1290,13 @@ public:
      */
     bool getElementPotentials(doublereal* lambda) const;
 
+    //! Indicates whether this phase type can be used with class MultiPhase for
+    //! equilibrium calculations. Returns `false` for special phase types which
+    //! already represent multi-phase mixtures, namely PureFluidPhase.
+    virtual bool compatibleWithMultiPhase() const {
+        return true;
+    }
+
     //@}
     /// @name Critical State Properties.
     /// These methods are only implemented by subclasses that implement

include/cantera/thermo/WaterSSTP.h

@@ -192,6 +192,10 @@ public:
 
     virtual doublereal satPressure(doublereal t);
 
+    virtual bool compatibleWithMultiPhase() const {
+        return false;
+    }
+
     //! Return the fraction of vapor at the current conditions
     /*!
      * Below Tcrit, this routine will always return 0, by definition of the

interfaces/cython/cantera/test/test_mixture.py

@@ -187,3 +187,8 @@ class TestMixture(utilities.CanteraTest):
             x.foobar = 300
         with self.assertRaises(AttributeError):
             x.foobar
+
+    def test_invalid_phase_type(self):
+        water = ct.Water()
+        with self.assertRaises(Exception):
+            self.mix = ct.Mixture([(self.phase1, 1.0), (water, 2.0)])

src/equil/MultiPhase.cpp

@@ -89,6 +89,11 @@ void MultiPhase::addPhase(ThermoPhase* p, doublereal moles)
                            "phases cannot be added after init() has been called.");
     }
 
+    if (!p->compatibleWithMultiPhase()) {
+        throw CanteraError("MultiPhase::addPhase", "Phase '{}'' is not "
+            "compatible with MultiPhase equilibrium solver", p->name());
+    }
+
     // save the pointer to the phase object
     m_phase.push_back(p);