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 96.87%. Comparing base (9f2a5be) to head (75e37e4).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2184   +/-   ##
=======================================
  Coverage   96.87%   96.87%           
=======================================
  Files         490      490           
  Lines       39462    39463    +1     
=======================================
+ Hits        38228    38229    +1     
  Misses       1234     1234           

Flag	Coverage Δ
unittests	96.87% <100.00%> (+<0.01%)	⬆️

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[ranocha]

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

[ranocha]

I converted this to draft mode so that nobody accidentally merges this breaking PR.