WIP: Fix bug in the semi-discrete entropy limiter

[amrueda]

In this PR, I propose a fix for the semi-discrete entropy limiter.

Thanks @weil4 for noticing the problem!

[github-actions]

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[bennibolm]

Is there another difference between the two new elixirs than just the volume flux?

[bennibolm]

If no, we could just adapt the volume flux in the tests and remove one of the elixirs.

[bennibolm]

Ah, I see that it is not even tested. Could you maybe add some test?

[amrueda]

The examples here are not meant to be tests (yet), but a proof of concept for debugging. They show that there is an error in the current implementation in the branch subcell-limiting: We get entropy production in these two tests with entropy_limiter_semidiscrete = true. That is wrong.

My "fix" causes entropy_limiter_semidiscrete = true to dissipate entropy in these two tests, but that is not enough to show that this is the only/correct fix that needs to be done. However, I don't know how to design a more definitive test.

[bennibolm]

I saw that two tests in tree_part2 are failing in this PR. Are those tests/e elixirs, where the entropy fix is used?

[amrueda]

I saw that two tests in tree_part2 are failing in this PR. Are those tests/e elixirs, where the entropy fix is used?

Yes, I'll update the reference values.