Add type-hints to LearnerND

[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 #377 (1160d48) into main (1b7e84d) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #377      +/-   ##
==========================================
+ Coverage   77.67%   77.76%   +0.08%     
==========================================
  Files          37       37              
  Lines        5393     5410      +17     
  Branches      981      981              
==========================================
+ Hits         4189     4207      +18     
  Misses       1058     1058              
+ Partials      146      145       -1     

Impacted Files	Coverage Δ
adaptive/learner/learnerND.py	59.40% <100.00%> (+0.67%)	⬆️
adaptive/learner/triangulation.py	86.28% <100.00%> (+0.48%)	⬆️
adaptive/types.py	100.00% <100.00%> (ø)
adaptive/learner/average_learner1D.py	74.00% <0.00%> (-0.67%)	⬇️
adaptive/learner/learner1D.py	87.16% <0.00%> (-0.50%)	⬇️
adaptive/tests/test_cquad.py	92.25% <0.00%> (+0.64%)	⬆️
adaptive/runner.py	69.28% <0.00%> (+0.68%)	⬆️

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