functionObjects/forces: limit the bins to handle moving meshes

This approach simply accumulates data outside the range of the bins into
the first or last bin which may be OK for small motions.  For larger
motions it may be better if the bins are updated or operate in a
coordinate system attached to the body for solid-body motion.
Resolves bug-report http://www.openfoam.org/mantisbt/view.php?id=1560