Bug 135 estimate range field

This is based (revision-history-wise) on the code in PR #144 (for issue #134), and is a minor extension of it to store 1/2 the bounding-box diagonal as a range estimate, the subject of issue #135. Only the final revision in the series ( dec4ba000335a551aeae059ac702acddfd8b72ff ) is specific to issue #135.