Catch case when windows do not span entire sequence for pair coalescence stats

[nspope]

Fixes issue reported here: #2986 (comment)

[nspope]

The issue was not passing TSK_REQUIRE_FULL_SPAN to tsk_treeseq_check_windows -- and I noticed that tsk_treeseq_divergence_matrix also doesn't do this here:

tskit/c/tskit/trees.c

Line 8463 in dcee409

ret = tsk_treeseq_check_windows(self, num_windows, windows, 0);

which may be a bug? (Not sure what is intended here)

Also would it be cleaner to move this option to the header?

tskit/c/tskit/trees.c

Line 1194 in dcee409

#define TSK_REQUIRE_FULL_SPAN 1

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.72%. Comparing base (dcee409) to head (2059859).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2989      +/-   ##
==========================================
- Coverage   89.72%   89.72%   -0.01%     
==========================================
  Files          29       29              
  Lines       31567    31567              
  Branches     6113     6113              
==========================================
- Hits        28324    28322       -2     
- Misses       1852     1853       +1     
- Partials     1391     1392       +1     

Flag	Coverage Δ
c-tests	86.55% <100.00%> (ø)
lwt-tests	80.78% <ø> (ø)
python-c-tests	88.98% <ø> (-0.03%)	⬇️
python-tests	99.01% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
c/tskit/trees.c	90.45% <100.00%> (ø)

... and 1 file with indirect coverage changes

[nspope]

This is ready whenever @jeromekelleher

[jeromekelleher]

Probably better to be consistent with other code on where the exception is coming from all right.

[nspope]

Done-- there's a number of other places in test_lowlevel.py where tskit.LibraryError is checked for rather than _tskit.LibraryError, but I left those alone here.