Fastslow initval

[surbhigoel77]

Description

The subdaily module uses an extended form of PModel called the FastSlowPModel. to estimate two categories of variables - fast variables (that respond to environmental changes instantaneously) and slow variables (that take time to respond to these changes). The module uses a memory_effect functions that facilitates the calculation of the slow variables as a combination of their lag values and optimal values.

In the current implementation of subdaily, at the start of the model run, the slow variables take the optimal value as the initial value and update the succeeding values using the:

$R{t} = R{t-1}(1 - \alpha) + O_{t} \alpha$

This PR intends to use an approach in which the last available realised value is used as the initial value for $x{i}$, $V_{max}$ and $J{max}$.

Fixes #82

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
• [ ] Tests added that prove fix is effective or that feature works

[davidorme]

This is a WIP, @surbhigoel77 ? Can you convert to a draft (link in Reviewers pane).

[davidorme]

Also - have you branched this off the feature/new-unit-tests branch, rather than develop? I think it is bringing the commits to that PR with it?

[surbhigoel77]

Also - have you branched this off the feature/new-unit-tests branch, rather than develop? I think it is bringing the commits to that PR with it?

Yes, I realised that when I pushed changes to fastslow-initval. The commit history shows changes from feature/new-unit-tests branch as well.

[davidorme]

No bother - it isn't a big deal 😄 I guess we could git revert those commits on this branch

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.24%. Comparing base (2072e0b) to head (9e3d36d). Report is 34 commits behind head on develop.

Files	Patch %	Lines
pyrealm/pmodel/subdaily.py	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #190 +/- ## ======================================== Coverage 95.24% 95.24% ======================================== Files 28 28 Lines 1703 1726 +23 ======================================== + Hits 1622 1644 +22 - Misses 81 82 +1 ```

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