Gauss-Legendre `DGSEM`: Open Points

Most elementary functionality of collocated Gauss-Legendre was added in https://github.com/trixi-framework/Trixi.jl/pull/1965 for 1D/2D periodic `TreeMesh` with `VolumeIntegralWeakForm()`.

This aims to list missing functionality, grouped by the mesh type:

## Planned

- [ ] `TreeMesh`
- [ ] Periodic
- [x] 1D periodic https://github.com/trixi-framework/Trixi.jl/pull/1965
- [x] 2D periodic https://github.com/trixi-framework/Trixi.jl/pull/1965
- [ ] 3D periodic
- [ ] Boundaries
- [x] 1D https://github.com/trixi-framework/Trixi.jl/pull/2827
- [x] 2D https://github.com/trixi-framework/Trixi.jl/pull/2841
- [ ] 3D
- [ ] Parabolic support
- [x] 1D https://github.com/trixi-framework/Trixi.jl/pull/2842
- [x] 2D https://github.com/trixi-framework/Trixi.jl/pull/2845
- [ ] 3D
- [ ] MPI
- [ ] 2D (for `TreeMesh` only 2D, no 3D)
- [ ] `P4estMesh`
- [ ] Periodic
- [x] 2D https://github.com/trixi-framework/Trixi.jl/pull/2847
- [ ] 3D
- [ ] Boundaries
- [ ] 2D
- [ ] Boundary Coordinates (Cartesian Meshes): https://github.com/trixi-framework/Trixi.jl/pull/2928
- [ ] Boundary Normal Vectors:
- [ ] 3D
- [ ] MPI
- [ ] 2D
- [ ] 3D
- [ ] Parabolic support
- [ ] 2D
- [ ] 3D
- [ ] `StructuredMesh`
- [ ] Periodic
- [x] 1D https://github.com/trixi-framework/Trixi.jl/pull/2828
- [ ] 2D
- [ ] 3D
- [ ] Boundaries
- [x] 1D https://github.com/trixi-framework/Trixi.jl/pull/2864
- [ ] 2D
- [ ] 3D
- [ ] Visualization
- [ ] Plots.jl
- [ ] Makie
- [ ] Trixi2Vtk.jl

## Not planned (right now)

- [ ] `UnstructuedMesh2D`
- [ ] `T8CodeMesh`
- [ ] `P4estMeshView` for coupled simulations with `SemidiscretizationCoupled`
- [ ] Flux-Differencing
- [ ] Non-Conservative Terms
- [ ] Shock-Capturing
- [ ] Non-conforming meshes (Mortars)
- [ ] AMR

Choose one unchecked capability and first compare it with the completed Gauss-Legendre work linked in this issue, especially PR 1965 and the later mesh-specific PRs. Confirm the selected mesh and dimension are in scope, then define completion as implementing that single item with the corresponding functionality validated; this issue does not name files or tests.

Written by the indexing model from the issue text.