Baseline data and code for two leaf model

[davidorme]

Description

This PR adds pyrealm_build_data/twoleaf, containing some input data files, some notebooks / scripts containing draft implementations of the Two Leaf model and then some parallel outputs to try and align development efforts. This new directory is likely to get reduced and revamped in the near future, but seemed like a good place to share stuff.

The key file is probably the two_leaf_python Myst Markdown notebook (can be run in jupyter-lab) which provides draft TwoLeafIrradiance and TwoLeafAssimilation classes that are structured to work with the existing PModel and SubdailyPModel classes (although it turns out there are some internal inconsistencies in those two class structures!).

Fixes #239

Type of change

• [x] 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

• [x] Make sure you've run the pre-commit checks: $ pre-commit run -a
• [ ] All tests pass: $ poetry run pytest

Further checks

• [x] 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 :white_check_mark:

Project coverage is 95.23%. Comparing base (290938a) to head (8d44792).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #240 +/- ## ======================================== Coverage 95.23% 95.23% ======================================== Files 28 28 Lines 1701 1701 ======================================== Hits 1620 1620 Misses 81 81 ```

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