Add StemAllometry class

[davidorme]

Description

This PR introduces the demography.t_model_functions StemAllometry class. This bundles the allometric equations into a higher level wrapper that takes a Flora | StemTraits instance and a set of stem sizes as DBH.

This new class then replaces the existing Community._calculate_t_model method and all of the rest of the existing code is updated to access stem allometry data through the class attributes. It also adds a simple test to confirm that the bundled allometry calculations still agree with the expected values tested on the individual functions.

Fixes #305

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 95.77%. Comparing base (1f315ba) to head (44498cf).
Report is 156 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #306      +/-   ##
===========================================
+ Coverage    95.29%   95.77%   +0.48%     
===========================================
  Files           28       34       +6     
  Lines         1720     2366     +646     
===========================================
+ Hits          1639     2266     +627     
- Misses          81      100      +19     

Flag	Coverage Δ
	95.77% <100.00%> (?)

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[MarionBWeinzierl]

This looks all sensible to me