ENH: minor consistency improvements for bounding box

- a valid() method (same as !empty() call) for consistency with other
  containers and data types

- a centre() method (same as midpoint() method) for consistency with
  other OpenFOAM geometric entities

src/OpenFOAM/algorithms/dynamicIndexedOctree/dynamicIndexedOctree.C

@@ -357,7 +357,7 @@ Foam::volumeType Foam::dynamicIndexedOctree<Type>::calcVolumeType
 
             subType = volumeType
             (
-                shapes_.getVolumeType(*this, subBb.midpoint())
+                shapes_.getVolumeType(*this, subBb.centre())
             );
         }