Update to t8code v4.0.0

[benegee]

• Initial run of generator.jl, based on include files from buildkite runs in [T8code] update to version 4 JuliaPackaging/Yggdrasil#12756.
• fixed.sh was cleaned up
• tests and examples adapted

[benegee]

Meanwhile, JuliaPackaging/Yggdrasil#12756 has been merged, i.e. we have binary packages here: https://github.com/JuliaBinaryWrappers/t8code_jll.jl

We now face the following issues:

1. Windows cannot be run in CI anymore because we excluded Windows from the BinaryBuilder builds. ✔️
2. MacOS was included, but fails in CI (some issue with libsc.3.0.0.dylib) https://github.com/DLR-AMR/T8code.jl/actions/runs/20780630626/job/59676973446?pr=105#step:5:146 (difficult to debug without a mac; plus mac is not officially supported by t8code, is it?)
3. Linux on x86 fails with a t8code error:

   AssertionError: Could not read cmesh from ascii version 2, but should be able to: cmesh/testfiles/test_msh_file_vers2_ascii.msh
   (I wonder if this could actually be investigated / fixed?)

4. Linux on x64 fails with earlier Julia versions, in particular
   • 1.6, 1.7 fail to precompile T8code due to missing libgfortran.so.5. This is presumably caused by the now enabled Fortran interface of t8code, which pulls in libgfortran, with a version that is higher than used by these earlier Julia versions.
   • 1.8 precompiles but causes a segmentation fault in the MPI tests https://github.com/DLR-AMR/T8code.jl/actions/runs/20779871199/job/59674755433#step:5:160

At least, Linux, x64, with Julia >= 1.9, runs at the moment and could be used to update downstream packages.

@spenke91 @sandro-elsweijer

[sandro-elsweijer]

Nice work!
2. Officially, we only support t8code on Linux with gcc. Since we have some Windows users (I think?), this should also work, but it is not tested in our CI. Same for Mac. We do not know about any Mac users and we also do not have a CI workflow for that.
3. Yeah we had some troubles with this. It is an IO error for Gmsh msh files from version 2. It is quite old and we plan to just drop the support. I will open an issue.

I cannot say anything about point 1 and 4

[JoshuaLampert]

Regarding 4: I think it would be fine to bump the minimal julia version for T8code.jl to v1.10 (and run CI on it) especially because Trixi.jl already has v1.10 as minimum required and it is the current LTS.

[benegee]

Thanks for your feedback!

Dropping support for all problematic platforms would be the easiest option and as a Trixi.jl person I would not be afraid to do so. I was just a little reluctant because of potential further downstream packages.

[JoshuaLampert]

What is the status of this PR, @benegee?