Allow for negative z offsets for shot origin

Follow up to !1215 (merged)

2 commits from @terencehill : 490abae3 and b18c702f

This MR replaces !1250 (closed), where the compression of shot origin offset range restrictions and compression were removed, because higher precision and range are not really needed and they would use 3 STATs instead of 1.

I included some cleanup and documentation.