Change how guide assignment model saves params from uns to var

[stefanpeidli]

PR Checklist

• Referenced issue is linked: Mixture Model stores arrays in .uns which don't allow the AnnData object to be written #722
• If you've fixed a bug :)
• Documentation in docs is updated: N/A

Description of changes

Instead of saving the guide assignment mixture model parameters to adata.uns, we now save to adata.var.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.25%. Comparing base (3e7f1ce) to head (919793f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #758      +/-   ##
==========================================
+ Coverage   72.19%   72.25%   +0.05%     
==========================================
  Files          47       47              
  Lines        5445     5449       +4     
==========================================
+ Hits         3931     3937       +6     
+ Misses       1514     1512       -2     

Files with missing lines	Coverage Δ
pertpy/preprocessing/_guide_rna.py	79.13% <100.00%> (+2.55%)	⬆️

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[Zethson]

Thank you!