snappyHexMesh::shellSurfaces: Added support for span refinement based on internal or external "closeness"

The closeness option in surfaceFeatures set in surfaceFeaturesDict, e.g.

    closeness
    {
        // Output the closeness of surface points to other surface elements.
        pointCloseness          yes;
    }

calculates and writes both the internal and external surface "closeness"
measures either of which could be used to set the span refinement in
snappyHexMesh depending on which side of the surface is being meshed which is
specified with either refinement mode "insideSpan" or "externalSpan", e.g. in
the tutorials/mesh/snappyHexMesh/pipe case the inside of the pipe is meshed and
refined based on the internal span using the following specification:

    refinementRegions
    {
        pipeWall
        {
            mode insideSpan;
            levels ((1000 2));
            cellsAcrossSpan 40;
        }
    }

tutorials/mesh/snappyHexMesh/pipe/system/snappyHexMeshDict

@@ -61,7 +61,7 @@ castellatedMeshControls
     {
         pipeWall
         {
-            mode span;
+            mode insideSpan;
             levels ((1000 2));
             cellsAcrossSpan 40;
         }