Fixing behaviour of T Model with zero DBH values

[davidorme]

Description

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Currently, if the DBH ($D$) of a stem is zero, then in the StemAllometry class:

• the crown fraction ($f_c$) is NaN, so...
• the sapwood mass (which uses $f_c$) is NaN

And in the StemAllocation class:

• Consequently, sapwood respiration is NaN and as a result NPP and all the delta variables (diameter, mass, foliage) are NaN.

That leads to all sorts of unexpected behaviour. We could make more sensible predictions when $D=0$ - which is what the first iteration of this PR did - but it basically doesn't make sense to make predictions when $D=0$. That's actually a bit more work because we currently allow zero so this PR:

• Adds a new _enforce_positive_sizes helper function to pyrealm.demography.tmodel
• Calls that function from within nearly all of the validation sections of the T model functions. We don't call it on the reproductive mass functions because that is an experimental extension that we want to be able to set to zero.
• Add tests that zero and negative values are caught correctly.
• Updates tests to pass

The PR also:

• Makes it a ValueError to create StemAllometry or Cohorts with negative DBH - this guarantees positive sizes and allows the per function validation to be skipped.
• Makes Cohorts.dbh_values a property raises ValueError when set with negative DBH values.

Fixes #524

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

Attention: Patch coverage is 98.48485% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.11%. Comparing base (e90a2d5) to head (4abea43).
Report is 9 commits behind head on develop.

Files with missing lines	Patch %	Lines
pyrealm/demography/tmodel.py	98.03%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #525      +/-   ##
===========================================
+ Coverage    96.02%   96.11%   +0.08%     
===========================================
  Files           41       41              
  Lines         3171     3242      +71     
===========================================
+ Hits          3045     3116      +71     
  Misses         126      126              

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