Subcell limiting: Improving interface for get_boundary_outer_state()

[bennibolm]

The subcell limiting on non-periodic domains requires boundary solution states to calculate subcell limiting bounds. For that, we implemented get_boundary_outer_state.

get_boundary_outer_state(u_inner, t,
                         boundary_condition,
                         normal_direction,
                         (direction,) # only for TreeMesh and StructuredMesh
                         mesh, equations,
                         dg, cache, indices...)

It requires some boundary information within the volume integral. That and its interface itself is somehow ugly and really messy.
#1954 added multiple new versions. After a discussion, we decided to add new versions to the elixir itself.

When there are more versions and/or we found a better way to fit this properly into Trixi, those functions can be saved in a more general file.

Right now, there are 4 versions of it in main:

• in solvers/dgsem_tree/dg_2d_subcell_limiting.jl:

  • for BoundaryConditionDirichlet

• in p4est_2d_dgsem/elixir_euler_supersonic_cylinder_sc_subcell.jl:

  • for boundary_condition_slip_wall, CompressibleEulerEquations2D and P4estMesh{2}
  • for boundary_condition_outflow and P4estMesh{2}
  • for boundary_condition_supersonic_inflow and P4estMesh{2}