Add Learner.new() method that returns an empty copy of the learner

[basnijholt]

Description

Add a method that returns a new instance of the Learner but without any data.

This is useful for testing and very useful when sending Learners over the network where its data is stored separately, e.g., in https://github.com/basnijholt/adaptive-scheduler.

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 #365 (0f77ca5) into master (914495c) will increase coverage by 0.01%.
The diff coverage is 91.42%.

@@            Coverage Diff             @@
##           master     #365      +/-   ##
==========================================
+ Coverage   80.20%   80.22%   +0.01%     
==========================================
  Files          37       37              
  Lines        5334     5359      +25     
  Branches      975      976       +1     
==========================================
+ Hits         4278     4299      +21     
- Misses        890      891       +1     
- Partials      166      169       +3     

Impacted Files	Coverage Δ
adaptive/learner/base_learner.py	88.37% <50.00%> (-1.88%)	⬇️
adaptive/learner/integrator_learner.py	88.32% <66.66%> (-0.20%)	⬇️
adaptive/learner/skopt_learner.py	59.42% <75.00%> (+0.95%)	⬆️
adaptive/learner/average_learner.py	84.03% <100.00%> (+0.27%)	⬆️
adaptive/learner/average_learner1D.py	77.66% <100.00%> (-0.53%)	⬇️
adaptive/learner/balancing_learner.py	75.34% <100.00%> (+0.34%)	⬆️
adaptive/learner/data_saver.py	87.09% <100.00%> (+0.43%)	⬆️
adaptive/learner/learner1D.py	89.52% <100.00%> (+0.05%)	⬆️
adaptive/learner/learner2D.py	78.89% <100.00%> (+0.13%)	⬆️
adaptive/learner/learnerND.py	60.40% <100.00%> (+0.14%)	⬆️
... and 5 more

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