Fix from_sep_smaller_min_nsources_cumul for trees without extent.

See merge request inducer/boxtree!37

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)

Fix list 3 documentation typo

See merge request inducer/boxtree!36

Fmmlib k rscale

See merge request inducer/boxtree!31

l^2 stickout tracking

See merge request inducer/boxtree!28

Fix target / source switch in list-3-close criterion (Fixes #16 on gitlab)

Closes #16

See merge request inducer/boxtree!30

Fix the description of list 4 in the logger.

See merge request !29

"separated bigger mpoles" should be "separated bigger source boxes".