Allowing incomplete days

[davidorme]

Description

This is a parallel implementation to #169 that was trying to unpack the test issues seen on that branch. The PR:

• Removes the restriction on complete days in creating a FastSlowScaler.
• Adds an internal padding attribute with FastSlowScaler that manages the wrapping of partial days so that they can be reshaped neatly onto days by observations when extracting daily acclimation data.

This also threw up some additional issues to do with handling missing data, giving rise to these changes:

• FastSlowScaler.get_daily_means now has an allow_partial_data option to handle missing data in the values being averaged to give a daily mean.
• The handle_nan option to the memory_effect function then handles missing data in the daily averages by holding over any previous valid value - this should be renamed allow_holdover to clarify the two roles.

The docs have been updated to reflect these changes, particularly the addition of a new section in the user docs on handling missing data in subdaily models.

Fixes #72

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 96.55172% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 94.88%. Comparing base (b41b416) to head (b2533f3).

Files	Patch %	Lines
pyrealm/pmodel/fast_slow_scaler.py	95.83%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #194 +/- ## =========================================== - Coverage 94.91% 94.88% -0.04% =========================================== Files 29 29 Lines 1535 1543 +8 =========================================== + Hits 1457 1464 +7 - Misses 78 79 +1 ```

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