New subdaily pmodel implementation

[davidorme]

Description

This PR builds on #172 to use the new OptimalChi methods to allow all approaches to be used in estimating a subdaily PModel. This radically simplifies the code compared to the current FastSlowPModel because all of the existing code within the pmodel module can be re-used more efficiently.

The PR:

• implements the new model class,
• adds a convenience conversion method to go from standard to subdaily models,
• adds a notebook showing the new approach,
• adds tests to show equivalence to the old approaches and to test the new interface with different optimal chi approaches.

Fixes #177 - but departs from the original plan that used an existing PModel as an input. This stands alone but provides the conversion function.

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
• [x] All tests pass: $ poetry run pytest

Further checks

• [x] Code is commented, particularly in hard-to-understand areas
• [x] Tests added that prove fix is effective or that feature works

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.83333% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 94.91%. Comparing base (8ace3d7) to head (6687d63).

Files	Patch %	Lines
pyrealm/pmodel/new_subdaily.py	94.73%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #183 +/- ## =========================================== + Coverage 93.80% 94.91% +1.11% =========================================== Files 28 29 +1 Lines 1468 1535 +67 =========================================== + Hits 1377 1457 +80 + Misses 91 78 -13 ```

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[tztsai]

Looks perfect to me.