snappyHexMesh: Simplified and rationalised the region refinement level specification

The inside or outside region refinement level is now specified using the simple
"level <level>" entry in refinementRegions e.g.

    refinementRegions
    {
        refinementBox
        {
            mode    inside;
            level   5;
        }
    }

rather than

    refinementRegions
    {
        refinementBox
        {
            mode    inside;
            levels  ((1E15 5));
        }
    }

where the spurious "1E15" number is not used and the '((...))' is unnecessary clutter.

tutorials/mesh/snappyHexMesh/flange/system/snappyHexMeshDict

@@ -131,8 +131,8 @@ castellatedMeshControls
     {
         refineHole
         {
-            mode inside;
-            levels ((1E15 3));
+            mode    inside;
+            level   3;
         }
     }
 

tutorials/mesh/snappyHexMesh/pipe/system/snappyHexMeshDict

@@ -61,8 +61,8 @@ castellatedMeshControls
     {
         pipeWall
         {
-            mode insideSpan;
-            levels ((1000 2));
+            mode    insideSpan;
+            level   (1000 2);
             cellsAcrossSpan 40;
         }
     }