Add omit_sites parameter to draw/svg

[hyanwong]

Fixes #2516

PR Checklist:

• Tests that fully cover new/changed functionality.
• Documentation including tutorial content if appropriate.
• Changelogs, if there are API changes.

[codecov]

Codecov Report

Merging #2519 (ead67f4) into main (bf37451) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2519   +/-   ##
=======================================
  Coverage   93.91%   93.91%           
=======================================
  Files          27       27           
  Lines       27193    27195    +2     
  Branches     1264     1265    +1     
=======================================
+ Hits        25537    25539    +2     
  Misses       1622     1622           
  Partials       34       34           

Flag	Coverage Δ
c-tests	92.24% <ø> (ø)
lwt-tests	89.05% <ø> (ø)
python-c-tests	73.17% <0.00%> (-0.02%)	⬇️
python-tests	98.95% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
python/tskit/trees.py	98.74% <ø> (ø)
python/tskit/drawing.py	98.99% <100.00%> (+<0.01%)	⬆️

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[benjeffery]

Thanks @hyanwong