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

applications/test/wallDist/Test-wallDist.C

@@ -67,7 +67,7 @@ int main(int argc, char *argv[])
 
     pointField newPoints(mesh.points());
 
-    const point half = meshBb.midpoint();
+    const point half = meshBb.centre();
 
     forAll(newPoints, pointi)
     {