PERK2 constructor consistence

[DanielDoehring]

This makes bS and cS keyword arguments for all PairedExplicitRK2 constructors.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.39%. Comparing base (824f7a8) to head (cd2e868).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2184      +/-   ##
==========================================
+ Coverage   96.37%   96.39%   +0.02%     
==========================================
  Files         483      483              
  Lines       38333    38320      -13     
==========================================
- Hits        36943    36938       -5     
+ Misses       1390     1382       -8     

Flag	Coverage Δ
unittests	96.39% <ø> (+0.02%)	⬆️

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[ranocha]

LGTM and can be merged when we're ready for a breaking release. Thanks!