Add 'day of week' effect as argument to Count observation process constructor

[cdc-mitzimorris]

Adds a day of week effect to a count observation process, per discussion in issue #712.

This PR includes implementation, unit tests, and tutorial. The tutorial uses the tutorial plot theme docs/tutorials/_tutorial_theme.py and therefore the docs won't render until PR #711 is approved and merged.

To implement this, I had Claude review the issue, the existing code base, and the way that day of week effects are calculated/used in pyrenew-multisignal. I reviewed all code edits manually and ran the tutorial notebook in jupyter.

[codecov]

Codecov Report

❌ Patch coverage is 92.59259% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.82%. Comparing base (923fda8) to head (e9e138d).

Files with missing lines	Patch %	Lines
pyrenew/model/multisignal_model.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #718      +/-   ##
==========================================
- Coverage   97.91%   97.82%   -0.09%     
==========================================
  Files          51       51              
  Lines        1580     1607      +27     
==========================================
+ Hits         1547     1572      +25     
- Misses         33       35       +2     

Flag	Coverage Δ
unittests	97.82% <92.59%> (-0.09%)	⬇️

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