Correct wrapping of particles

This fixes the issue discussed last Friday. In summary:

• Box-wrapping should occur for [0, box_size[ and not [0, box_size].
• The splitting of cells should obey the same interval rules.

This solves part of the bugs found by @bvandenbroucke.

LGTM, reassigning to @pdraper for approval.

assigned to @pdraper

OK for me as well. Accepting.

merged

mentioned in commit f1e7b821