AverageLearner: fix zero mean

[basnijholt]

Description

Fixes #275

Checklist

• Fixed style issues using pre-commit run --all (first install using pip install pre-commit)
• pytest passed

Type of change

Check relevant option(s).

• 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

[basnijholt]

I have enabled Auto-merge.

[codecov-commenter]

Codecov Report

Merging #276 (9a3f8b9) into master (4e34aba) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #276      +/-   ##
==========================================
+ Coverage   80.46%   80.56%   +0.10%     
==========================================
  Files          35       35              
  Lines        4653     4662       +9     
  Branches      838      839       +1     
==========================================
+ Hits         3744     3756      +12     
+ Misses        783      782       -1     
+ Partials      126      124       -2     

Impacted Files	Coverage Δ
adaptive/learner/average_learner.py	86.31% <100.00%> (+0.60%)	⬆️
adaptive/tests/test_average_learner.py	95.12% <100.00%> (+0.67%)	⬆️
adaptive/learner/learner1D.py	89.74% <0.00%> (+0.28%)	⬆️
adaptive/runner.py	71.73% <0.00%> (+0.70%)	⬆️

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