basicThermo: Cache thermal conductivity kappa rather than thermal diffusivity alpha

Now that Cp and Cv are cached it is more convenient and consistent and slightly
more efficient to cache thermal conductivity kappa rather than thermal
diffusivity alpha which is not a fundamental property, the appropriate form
depending on the energy solved for.  kappa is converted into the appropriate
thermal diffusivity for the energy form solved for by dividing by the
corresponding cached heat capacity when required, which is efficient.

tutorials/multiphase/compressibleInterFoam/laminar/cylinder/constant/cloudProperties

@@ -42,7 +42,7 @@ solution
         thermo:mu.air   cell;
         T.air           cell;
         Cp.air          cell;
-        kappa           cell;
+        thermo:kappa.air cell;
         p               cell;
     }