DG Multi with FDSBP has incompatible data types for visualization

When running a simulation with DG Multi and SBP operators, it returns the following data types: SBP Weak form ```julia typeof(u) = RecursiveArrayTools.VectorOfArray{Float64, 2, StructArrays.StructVector{SVector{4, Float64}, NTuple{4, Vector{Float64}}, Int64}} typeof(parent(u)) = StructArrays.StructVector{SVector{4, Float64}, NTuple{4, Vector{Float64}}, Int64} ``` SBP Flux-differencing ```julia typeof(u) = RecursiveArrayTools.VectorOfArray{Float64, 2, Vector{SVector{4, Float64}}} typeof(parent(u)) = Vector{SVector{4, Float64}} ``` The second data type indeed is incompatible with [PlotTriangulated2D](https://github.com/trixi-framework/Trixi.jl/blob/main/src/visualization/types.jl#L331), for the function call `plot(sol)`. A quick fix is adding: ```julia u = VectorOfArray(StructArray(parent(u))) ``` before returning the RHS of the function call [PlotData2D DGMulti](https://github.com/trixi-framework/Trixi.jl/blob/main/src/visualization/types.jl#L688). That of course breaks the previous case.