Add type-hints to Learner2D

[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 #375 (ab976a9) into main (8fd6de5) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@ Coverage Diff @@## main #375 +/- ##
==========================================
- Coverage 77.69% 77.65% -0.05% 
==========================================
Files 37 37 Lines 5420 5423 +3 Branches 982 982 ==========================================
Hits 4211 4211 - Misses 1062 1064 +2 - Partials 147 148 +1 

Impacted Files	Coverage Δ
adaptive/tests/test_pickling.py	88.67% <ø> (ø)
adaptive/learner/learner2D.py	76.52% <100.00%> (+0.22%)	⬆️
adaptive/tests/test_runner.py	86.25% <100.00%> (ø)
adaptive/learner/learner1D.py	86.73% <0.00%> (-0.99%)	⬇️
adaptive/tests/test_cquad.py	92.25% <0.00%> (+0.64%)	⬆️

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