Minor updates to T Model functions and testing

[davidorme]

This PR is to:

• Adds a T Model function to reverse calculate DBH from stem height.
• Moves the R T model data into tests/unit/demography/conftest.py so it can be shared and splits it into a dataframe of expected values and a new Flora object created from the PFT definitions.
• Updates the unit testing for the T Model functions to use these new structures.

Fixes #302 (issue)

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 95.71%. Comparing base (1f315ba) to head (e431d04).
Report is 154 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #303      +/-   ##
===========================================
+ Coverage    95.29%   95.71%   +0.42%     
===========================================
  Files           28       34       +6     
  Lines         1720     2336     +616     
===========================================
+ Hits          1639     2236     +597     
- Misses          81      100      +19     

Flag	Coverage Δ
	95.71% <100.00%> (?)

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