Add type-hints to BalancingLearner

[basnijholt]

Description

Please include a summary of the change and which (if so) issue is fixed.

Fixes #(ISSUE_NUMBER_HERE)

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)
• (Code) style fix or documentation update
• This change requires a documentation update

[codecov-commenter]

Codecov Report

Merging #371 (afc2b5f) into main (1b7e84d) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #371      +/-   ##
==========================================
- Coverage   77.67%   77.65%   -0.03%     
==========================================
  Files          37       37              
  Lines        5393     5401       +8     
  Branches      981      981              
==========================================
+ Hits         4189     4194       +5     
- Misses       1058     1059       +1     
- Partials      146      148       +2     

Impacted Files	Coverage Δ
adaptive/learner/balancing_learner.py	71.74% <100.00%> (+1.05%)	⬆️
adaptive/learner/average_learner1D.py	73.66% <0.00%> (-1.00%)	⬇️
adaptive/learner/learner1D.py	86.91% <0.00%> (-0.75%)	⬇️
adaptive/learner/learnerND.py	59.10% <0.00%> (+0.37%)	⬆️
adaptive/tests/test_cquad.py	92.25% <0.00%> (+0.64%)	⬆️

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