Naive fix for too high metabolic fractions

[jacobcook1995]

Description

This PR is a naive fix for metabolic fractions of litter input being too high, basically be converting the existing test to a step that forces the value to conform. This will be handled more elegantly when I get round to addressing #1010, but I think this quick fix is good enough for now.

I've also changed what this function raises errors for, so that it now checks for lignin proportions outside the physically possible range, which would break the logic of the function.

Fixes #1359

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 94.99%. Comparing base (d8e74f4) to head (bc556af).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1360      +/-   ##
===========================================
+ Coverage    94.95%   94.99%   +0.03%     
===========================================
  Files           71       71              
  Lines         7369     7366       -3     
===========================================
  Hits          6997     6997              
+ Misses         372      369       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.