Small fixes to prevent negative microbial populations from propagating further errors

[jacobcook1995]

Description

Two changes here.

1. Negative microbial populations now no longer lose a negative amount of biomass to maintenance. Instead the biomass loss rate is zero
2. The animal available microbial consumption pools now treat any negative microbial pool as zero (previously this only happened for the two mycorrhizal pools)

These fixes are needed not just for the mycorrhizal problems (which a better long term solution for is described in #1197), but also because animal consumption will be potentially able to drive microbial pools negative.

Fixes #1198

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 95.00%. Comparing base (6407711) to head (326f48c).
⚠️ Report is 8 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1200   +/-   ##
========================================
  Coverage    95.00%   95.00%           
========================================
  Files           71       71           
  Lines         7269     7269           
========================================
  Hits          6906     6906           
  Misses         363      363           

☔ 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.

[vgro]

LGTM