Add 1D `DGMulti` `GaussSBP` solver

[jlchan]

GaussSBP solvers previously were only implemented in 2D and 3D. This PR adds 1D capabilities (using that sum factorization just reduces to mul! in 1D).

[github-actions[bot]]

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Testing

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Created with :heart: by the Trixi.jl community.

[jlchan]

Thanks for the quick review!

[ranocha]

Codecov is failing but I would like to see whether CI covers the new code...

[ranocha]

Maybe you can push this branch to a branch in the main repo and open a new PR so that we don't have the problem with codecov?

[jlchan]

Maybe you can push this branch to a branch in the main repo and open a new PR so that we don't have the problem with codecov?

Sure, no problem

[jlchan]

Superceded by #1932