Make LearnerND pickleable

[basnijholt]

Description

This makes the LearnerND pickleable.

Other learners have been made pickleable in #264.

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)
• This change requires a documentation update

[codecov-commenter]

Codecov Report

Merging #272 (90b250b) into master (c820704) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #272 +/- ##
==========================================
- Coverage 80.72% 80.72% -0.01% 
==========================================
Files 38 38 Lines 5018 5022 +4 Branches 918 918 ==========================================
+ Hits 4051 4054 +3 - Misses 822 823 +1 
Partials 145 145 

Impacted Files	Coverage Δ
adaptive/tests/test_pickling.py	88.67% <ø> (ø)
adaptive/learner/base_learner.py	84.09% <100.00%> (ø)
adaptive/learner/learnerND.py	60.19% <100.00%> (+0.50%)	⬆️
adaptive/learner/average_learner1D.py	78.88% <0.00%> (-0.75%)	⬇️

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