return mean vertical flow for all layers

[vgro]

This PR returns mean vertical_flow for all soil layers.

Fixes #891

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 94.08%. Comparing base (18686f7) to head (b68ecc9).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #930      +/-   ##
===========================================
- Coverage    94.08%   94.08%   -0.01%     
===========================================
  Files           78       78              
  Lines         6172     6171       -1     
===========================================
- Hits          5807     5806       -1     
  Misses         365      365              

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[jacobcook1995]

LGTM!