Assure conservation for SSP scheme

[bennibolm]

This PR corrects the implementation of the SSPRK33 method. It mimics the implementation of OrdinaryDiffEq.jl to guarantee conservation.

Before, the convex combination of the third stage of SSP33 looked like:
@. integrator.u = 1/3 * integrator.r0 + 2/3 * integrator.u
This led to problems with conservation in machine precision. The errors in conservation grow slightly over time:

Now, with using @. integrator.u = (1 * integrator.r0 + 2 * integrator.u) / 3 the errors level off.

These are results of a blast wave simulation with 64x64 elements using positivity subcell limiting for density.

[github-actions]

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[sloede]

I can't believe that it was this, that this was the straw that broke the camel's back 😱 Great catch! Just two suggestions to improve clarity, then it can be merged (or ignore the suggestions and we merge it as it is)

[bennibolm]

I can't believe that it was this, that this was the straw that broke the camel's back 😱 Great catch!

Yes, I felt the same way. Thanks to you for sending me the working code (even I checked it before). It animated me again to just copy their complete code.

[sloede]

LGTM!

[sloede]

One thing I forgot to say: What's missing in the code right now is a comment explaining why you did have to implement it the way you did, with a reference to this PR. You can either do it here (if the PR hasn't automerged by then), or in any of your subsequent PRs.