Fix from_sep_smaller_min_nsources_cumul for trees without extent.

When extents were turned off, a nonzero from_sep_smaller_min_nsources_cumul would lead to boxes being silently ignored during interaction list construction.

This change fixes this problem and also adds a test for from_sep_smaller_min_nsources_cumul.

Related: This caused pytential#83 (closed) (which turned out to be a real bug and not a cache problem)