Handle NaNs when calculating max difference in `Splash.estimate_initial_soil_moisture`

[tztsai]

Description

I replaced np.max with np.nanmax when calculating the maximum difference between the start of year soil moisture and the final day of the year and then update the start point to the end of year in the estimate_initial_soil_moisture method of the Splash model, to output more meaningful error messages when running this method.

Fixes #192

Type of change

• [x] New feature (non-breaking change which adds functionality)

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

[tztsai]

Shouldn't the function inform whether the iteration has reached equilibrium before returning the array?

[davidorme]

Yeah - that would be useful - we should add a warning to flag that the returned sequence doesn't meet the convergence criteria.

[davidorme]

Caught that bug that was causing the tests to fail.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 74.07407% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 94.58%. Comparing base (8f7d032) to head (7626476).

Files	Patch %	Lines
pyrealm/splash/splash.py	74.07%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #193 +/- ## =========================================== - Coverage 94.86% 94.58% -0.28% =========================================== Files 28 28 Lines 1499 1514 +15 =========================================== + Hits 1422 1432 +10 - Misses 77 82 +5 ```

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