externalTemperatureFvPatchScalarField: Changed fixesValue() to true

With the change the boundary temperature is updated only via the evaluate call
rather than derived from the corresponding energy solution.  This has proved
more stable and convergent for the wallBoiling cases.

src/ThermophysicalTransportModels/coupledThermophysicalTransportModels/externalTemperature/externalTemperatureFvPatchScalarField.H

@@ -240,10 +240,12 @@ public:
 
         // Access
 
-            //- Allow manipulation of the boundary values
+            //- Disallow manipulation of the boundary values
+            //  The boundary temperature is evaluated only by
+            //  this boundary condition
             virtual bool fixesValue() const
             {
-                return false;
+                return true;
             }