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.

applications/utilities/preProcessing/applyBoundaryLayer/createFields.H

@@ -38,7 +38,7 @@ volVectorField U
 );
 
 Info<< "Calculating wall distance field" << endl;
-volScalarField y(wallDist(mesh).y());
+const volScalarField& y(wallDist::New(mesh).y());
 
 // Set the mean boundary-layer thickness
 dimensionedScalar ybl("ybl", dimLength, 0);