time windows for general_stat (branch mode)

[petrelharp]

This is a WIP; for now I'm just moving the python implementation in tests/test_tree_stats.py over here from #3266.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.89%. Comparing base (1d8f453) to head (b11b7a9).
⚠️ Report is 1 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (1d8f453) and HEAD (b11b7a9). Click for more details.

HEAD has 6 uploads less than BASE

Flag	BASE (1d8f453)	HEAD (b11b7a9)
python-tests	6	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3271       +/-   ##
===========================================
- Coverage   89.75%   78.89%   -10.86%     
===========================================
  Files          29       29               
  Lines       32545    32545               
  Branches     5962     5962               
===========================================
- Hits        29211    25677     -3534     
- Misses       1890     5322     +3432     
- Partials     1444     1546      +102     

Flag	Coverage Δ
c-tests	86.76% <ø> (ø)
lwt-tests	80.38% <ø> (ø)
python-c-tests	88.24% <ø> (ø)
python-tests	?
python-tests-no-jit	33.02% <ø> (ø)
python-tests-numpy1	50.71% <ø> (ø)

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see 13 files with indirect coverage changes

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