multiphaseEulerFoam: Add "none" diameterModel for phases that are always continuous

This model will generate an error if the diameter is requested. This
will happen if another sub model is included that depends on the
diameter of the continuous phase. It therefore provides a check that the
sub-modelling combination is valid.

Patch contributed by Institute of Fluid Dynamics,
Helmholtz-Zentrum Dresden - Rossendorf (HZDR)

tutorials/multiphase/multiphaseEulerFoam/RAS/fluidisedBed/constant/phaseProperties

@@ -38,11 +38,7 @@ air
 {
     type          purePhaseModel;
 
-    diameterModel constant;
-    constantCoeffs
-    {
-        d               1;
-    }
+    diameterModel none;
 
     residualAlpha   0;
 }