Make subject parameter mandatory

[fepegar]

Fixes #1020.

Description

The example in GridSampler was not working, as the subject parameter is actually mandatory. This PR fixes the example and the behavior of the class.

Checklist

• I have read the CONTRIBUTING docs and have a developer setup (especially important are pre-commitand pytest)
• Non-breaking change (would not break existing functionality)
• Breaking change (would cause existing functionality to change)
• Tests added or modified to cover the changes
• Integration tests passed locally by running pytest
• In-line docstrings updated
• Documentation updated, tested running make html inside the docs/ folder
• This pull request is ready to be reviewed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.47%. Comparing base (fd578e3) to head (860d533).
Report is 110 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1029      +/-   ##
==========================================
+ Coverage   86.36%   86.47%   +0.11%     
==========================================
  Files          91       91              
  Lines        5773     5769       -4     
==========================================
+ Hits         4986     4989       +3     
+ Misses        787      780       -7     

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[fepegar]

@all-contributors please add @arthursw for bug, doc

[allcontributors]

@fepegar

I've put up a pull request to add @arthursw! 🎉