wallDist: now a MeshObject cached and updated automatically with a run-time selected algorithm

When using models which require the wallDist e.g. kOmegaSST it will
request the method to be used from the wallDist sub-dictionary in
fvSchemes e.g.

wallDist
{
    method meshWave;
}

specifies the mesh-wave method as hard-coded in previous OpenFOAM versions.

tutorials/compressible/rhoPimpleFoam/ras/cavity/system/fvSchemes

@@ -63,5 +63,10 @@ fluxRequired
     p               ;
 }
 
+wallDist
+{
+    method meshWave;
+}
+
 
 // ************************************************************************* //