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[bot]]

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[codecov[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1958/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1958/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `96.11% <88.46%> (-<0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework#carryforward-flags-in-the-pull-request-comment) to find out more.

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