Replace ZeroSumNormal with Normal for _annual_seasonal

[maresb]

I'm pretty sure you want to use Normal here instead of ZeroSumNormal. I can explain more later.

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📚 Documentation preview 📚: https://causalpy--679.org.readthedocs.build/en/679/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.35%. Comparing base (a67408a) to head (75a924b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #679   +/-   ##
=======================================
  Coverage   94.35%   94.35%           
=======================================
  Files          44       44           
  Lines        7517     7517           
  Branches      456      456           
=======================================
  Hits         7093     7093           
  Misses        262      262           
  Partials      162      162           

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[drbenvincent]

Thanks for this. Would you be able to put in a very brief explanation in the docstring (or code comment) so that this doesn't end up getting reverted in the future?