ENH: adjustments to searchable surfaces

- code reduction, documentation, code stubs for spheroid (#1901)

- make searchableSurfaceCollection available as 'collection'
  for consistency with other objects

tutorials/mesh/snappyHexMesh/motorBike_leakDetection/system/snappyHexMeshDict

@@ -43,7 +43,7 @@ geometry
 
     refinementBox
     {
-        type searchableBox;
+        type box;
         min (-1.0 -0.7 0.0);
         max ( 8.0  0.7 2.5);
     }