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 inducer/pytential#83 (which turned out to be a
real bug and not a cache problem)