Add StemAllocation class

[davidorme]

Description

Very similar to the last PR #305!

This PR introduces the demography.t_model_functions StemAllocation class. This bundles the allocation equations from the T Model into a higher level wrapper that takes:

• a Flora | StemTraits instance to provide stem traits,
• a StemAllometry instance that provides stem size and scaling calculations, and
• an array of potential GPP values to be allocated to respiration, turnover and growth.

This class isn't used yet in the demography simulation workflow but will be. It also works as a user-facing visualisation tool. The PR also adds a simple test to confirm that the bundled allocation calculations still agree with the expected values tested on the individual functions.

Fixes #307

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.84%. Comparing base (1f315ba) to head (fabf312).
Report is 164 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #308      +/-   ##
===========================================
+ Coverage    95.29%   95.84%   +0.54%     
===========================================
  Files           28       34       +6     
  Lines         1720     2404     +684     
===========================================
+ Hits          1639     2304     +665     
- Misses          81      100      +19     

Flag	Coverage Δ
	95.84% <100.00%> (?)

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[MarionBWeinzierl]

looks good to me