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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