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/opposite_walls/system/snappyHexMeshDict

@@ -40,7 +40,7 @@ geometry
     }
     all
     {
-        type searchableBox;
+        type box;
         min (-1000 -1000 -1000);
         max (1000 1000 1000);
     }