Unpin the update interval in hydrology

[davidorme]

Description

This PR:

• Relaxes the hydrology model to allow intervals other than 30 days and adds code to calculate the actual number of days in the update interval. It warns when the update interval is not a whole number of days.
• Updates the docs to describe the internal calculation.
• Updates the tests - there was a test that using 1 week would fail - I've retained the 1 week test case but updated it to show that it works. The resulting test values seem sensible: the model gets the same precipitation in fewer days, so the vertical flow and runoff are higher, but there is less time for soil evaporation, which is lower.

There are also a couple of requires mypy fixes - the numpy plugin for mypy has been deprecated and the new system picks up a couple of things.

@jacobcook1995 There are a couple of weird typing things here from litter: two functions that claimed to return dict[str, NDArray] but didn't and hadn't been caught for some reason.

Fixes #980

Type of change

• 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

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

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works
• Relevant documentation reviewed and updated

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.23%. Comparing base (5b2cc1b) to head (d9d13f3).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #981      +/-   ##
===========================================
- Coverage    94.23%   94.23%   -0.01%     
===========================================
  Files           79       79              
  Lines         6589     6587       -2     
===========================================
- Hits          6209     6207       -2     
  Misses         380      380              

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[jacobcook1995]

LGTM! (Honestly really surprised that that litter model typing ever passed CI)

[vgro]

LGTM!

Could you please delete the TODO in line 18 in hydrology_model.py