corrected calculation of midpoints

[vgro]

This PR updates the caluclation of layer midpoints.

Fixes #1363

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works
• Relevant documentation reviewed and updated

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.85%. Comparing base (d6d8258) to head (e2ddcf5).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1364      +/-   ##
===========================================
- Coverage    94.85%   94.85%   -0.01%     
===========================================
  Files           71       71              
  Lines         7388     7387       -1     
===========================================
- Hits          7008     7007       -1     
  Misses         380      380              

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[jacobcook1995]

LGTM!