ENH: leak-detection. Fixes #1572.

The final leak can only be decided once all cells have been
deleted. So only exit on final invocation and give warning-only
beforehand. This avoids a lot of false positives.
The tutorial itself didn't actually produce a mesh with leakage
with the old settings. Upped the refinement level to force it
to go through the hole in the geometry.

tutorials/mesh/snappyHexMesh/motorBike_leakDetection/system/snappyHexMeshDict

@@ -119,7 +119,7 @@ castellatedMeshControls
         "motorBike.*"
         {
             // Surface-wise min and max refinement level
-            level (5 6);
+            level (6 6);
 
             // Optional specification of patch type (default is wall). No
             // constraint types (cyclic, symmetry) etc. are allowed.