Learner1D type hints and add typeguard to pytest tests

[basnijholt]

Description

I am splitting up #244 into smaller parts.

Since it has been ready for review for almost 2 years but no-one has looked at it, I will go ahead and auto-complete this as soon as the typeguard and mypy issues have been solved.

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 #325 (087a00f) into master (c820704) will increase coverage by 0.04%.
The diff coverage is 97.46%.

@@ Coverage Diff @@## master #325 +/- ##
==========================================
+ Coverage 80.72% 80.77% +0.04% 
==========================================
Files 38 38 Lines 5018 5045 +27 Branches 918 918 ==========================================
+ Hits 4051 4075 +24 - Misses 822 824 +2 - Partials 145 146 +1 

Impacted Files	Coverage Δ
adaptive/learner/learner1D.py	90.15% <97.22%> (+0.41%)	⬆️
adaptive/learner/average_learner1D.py	78.88% <100.00%> (-0.75%)	⬇️
adaptive/learner/base_learner.py	84.09% <100.00%> (ø)
adaptive/tests/test_learners.py	98.83% <100.00%> (+<0.01%)	⬆️

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