Write subcanopy biomasses to data during PlantMode.__init__

[davidorme]

Description

This PR:

• Updates the Subcanopy.__init__ method to include writing the subcanopy pool CNP arrays to data. This is so that these arrays are available during AnimalModel.__init__ for the setup of ArrayResources (Array resources implementation #1318).
• Fixes a misleading error message from the variables module.
• Fixes a bug in the Subcanopy - the seedbank biomass was replacing the vegetation biomass.

Fixes #1333

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 94.95%. Comparing base (0e7fd6a) to head (26f45a1).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1335   +/-   ##
========================================
  Coverage    94.95%   94.95%           
========================================
  Files           71       71           
  Lines         7369     7379   +10     
========================================
+ Hits          6997     7007   +10     
  Misses         372      372           

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[sallymatson]

LGTM