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%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more