Docs for __getitem__(), __contains__(), and migration_matrices()

[grahamgower]

No description provided.

[codecov]

Codecov Report

Merging #320 (b1546f8) into main (434622f) will not change coverage.
The diff coverage is 100.00%.

❗ Current head b1546f8 differs from pull request most recent head 0893d80. Consider uploading reports for the commit 0893d80 to get more accurate results

@@           Coverage Diff           @@
##             main     #320   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files           4        4           
  Lines        1245     1245           
=======================================
  Hits         1244     1244           
  Misses          1        1           

Impacted Files	Coverage Δ
demes/demes.py	100.00% <100.00%> (ø)

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[grahamgower]

Unless anyone has a problem with the proposed API for migration_matrices(), I'd like to push out another release. (But no major rush, it doesn't have to be today).