Make the default margins accessible as attributes

[hyanwong]

Trivial, but helps when e.g. calculating point positions on the plot, which I'm doing for e.g. making tangle grams. No need to document (yet)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.62%. Comparing base (1a1e75e) to head (da18351).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3196   +/-   ##
=======================================
  Coverage   89.62%   89.62%           
=======================================
  Files          28       28           
  Lines       31971    31975    +4     
  Branches     5873     5873           
=======================================
+ Hits        28655    28659    +4     
  Misses       1886     1886           
  Partials     1430     1430           

Flag	Coverage Δ
c-tests	86.66% <ø> (ø)
lwt-tests	80.38% <ø> (ø)
python-c-tests	88.18% <ø> (ø)
python-tests	98.86% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
python/tskit/drawing.py	98.35% <100.00%> (+<0.01%)	⬆️

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