chore: add codecov.yaml and exclude legacy files

[chanind]

Description

We have a number of legacy files which we may want to delete, but at least we shouldn't be including them in code coverage stats as it makes the project look like it's not properly tested. This PR adds a codecov.yaml where we can further configure codecov, and excludes the following files:

• sae_lens/analysis/feature_statistics.py
• sae_lens/analysis/tsea.py
• sae_lens/toolkit/pretrained_saes.py

These files can probably be deleted in the future, they're not documented or referenced anywhere except some legacy jupyter notebooks.

Type of change

Please delete options that are not relevant.

• Chore
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.46%. Comparing base (fecfe5d) to head (ffb2e85).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #372      +/-   ##
==========================================
+ Coverage   67.09%   72.46%   +5.37%     
==========================================
  Files          25       22       -3     
  Lines        3519     3258     -261     
  Branches      457      431      -26     
==========================================
  Hits         2361     2361              
+ Misses       1028      767     -261     
  Partials      130      130              

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[chanind]

merging as this is just affects codecov stats, which should be pretty non-controversial