Refactor PERK2 time integration method to use fixed time step

[warisa-r]

As mentioned by @ranocha in #1908 , a user is now able to set a fixed time step for simulations without a stepsize callback for PERK2 time integration method. The implementation is from @bennibolm in #1919.

[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 88.46154% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 96.11%. Comparing base (3b52a30) to head (8a1922c).

Files	Patch %	Lines
...ation/paired_explicit_runge_kutta/methods_PERK2.jl	88.46%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1958      +/-   ##
==========================================
- Coverage   96.11%   96.11%   -0.00%     
==========================================
  Files         460      460              
  Lines       36926    36947      +21     
==========================================
+ Hits        35490    35509      +19     
- Misses       1436     1438       +2     

Flag	Coverage Δ
unittests	96.11% <88.46%> (-<0.01%)	⬇️

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[bennibolm]

LGTM. Thanks