Add tests for index_matrix and index_vector

[kklein]

Checklist

• Added a CHANGELOG.rst entry

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.73%. Comparing base (0d1958c) to head (9c23360).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
+ Coverage   94.46%   94.73%   +0.27%     
==========================================
  Files          15       15              
  Lines        1806     1806              
==========================================
+ Hits         1706     1711       +5     
+ Misses        100       95       -5     

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