Refactor optimisation args

[BradyPlanden]

Description

• Moves common Pints' optimisation keys from kwargs to optional args for improved autocomplete and IDE highlighting
• Args max_iterations and population_size to default args across optimisers, with maxiter and popsize only available on SciPy based optimisers
• BasePintsOptimiser.pints_optimiser renamed to BasePintsOptimiser.optimiser
• Improves docstrings for optimiser classes

Issue reference

Fixes # (issue-number)

Review

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.14%. Comparing base (c8b00e6) to head (4667641).
Report is 14 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #551      +/-   ##
===========================================
+ Coverage    99.08%   99.14%   +0.05%     
===========================================
  Files           52       55       +3     
  Lines         3605     3848     +243     
===========================================
+ Hits          3572     3815     +243     
  Misses          33       33              

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[BradyPlanden]

Marked as 'show', but would appreciate your input @NicolaCourtier.