Add support for P4estMesh to subcell-limiting

[bennibolm]

Adding support for subcell limiting with P4estMesh.

TODOs:

• Check if StructuredMesh and P4estMesh give the same results
  -> Different order in calculating of the boundary fluxes yields slightly different (machine precision) results

[github-actions]

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[bennibolm]

I'm thinking about whether it's useful to keep this default implementation of get_boundary_outer_state that only returns the inner value. When keeping it, the user doesn't really get an information if the used boundary condition has an influence to the bounds calculation or not.

Trixi.jl/src/solvers/dgsem_tree/dg_2d_subcell_limiters.jl

Lines 1853 to 1858 in 1b2ce66

	# Default implementation of `get_boundary_outer_state` returns inner value.
	@inline function get_boundary_outer_state(u_inner, cache, t, boundary_condition,
	orientation_or_normal, direction, equations,
	dg, indices...)
	return u_inner
	end

What do you think about that @amrueda ?

EDIT: On the other hand, it is useful for boundary conditions like boundary_condition_outflow.

[amrueda]

I'm thinking about whether it's useful to keep this default implementation of get_boundary_outer_state that only returns the inner value. When keeping it, the user doesn't really get an information if the used boundary condition has an influence to the bounds calculation or not.

Trixi.jl/src/solvers/dgsem_tree/dg_2d_subcell_limiters.jl

Lines 1853 to 1858 in 1b2ce66

	# Default implementation of `get_boundary_outer_state` returns inner value.
	@inline function get_boundary_outer_state(u_inner, cache, t, boundary_condition,
	orientation_or_normal, direction, equations,
	dg, indices...)
	return u_inner
	end

What do you think about that @amrueda ?

True, I was also thinking recently about this routine. I think it is better to remove it because it can lead a user to think that the bound computation takes into account their BCs. However, sometimes it makes sense to just use the inner solution. Is it possible to activate this computation with an input argument and keep it inactive as default?

[bennibolm]

True, I was also thinking recently about this routine. I think it is better to remove it because it can lead a user to think that the bound computation takes into account their BCs. However, sometimes it makes sense to just use the inner solution. Is it possible to activate this computation with an input argument and keep it inactive as default?

Yesterday, I revised the implementation of this routine. It is much more flexible and cleaner now.
Yes, it would be possible to add a boolean variable like use_inner_value_as_boundary_value_as_default (or something longer 😉 ) and throwing an error when the default routine is clled but the variable is false. But actually I think this is not very nice and we would have another stupid parameter 😣
So, I think I will just delete the default routine. If someone still wants to just use the inner value it is about 4 lines of code to add to the elixir.

[amrueda]

LGTM!
Just a minor comment: