WIP: Sc/p4est view coupled hanging nodes

[SimonCan]

Added capability of coupling pest mesh views with hanging/non-conforming nodes.

Convergence test for examples/p4est_2d_dgsem/elixir_advection_coupled.jl:

####################################################################################################
l2
scalar (semi 1)     scalar (semi 2)     
error     EOC       error     EOC       
1.33e-04  -         1.33e-04  -         
7.82e-06  4.09      7.82e-06  4.09      
4.93e-07  3.99      4.93e-07  3.99      

mean      4.04      mean      4.04      
----------------------------------------------------------------------------------------------------
linf
scalar (semi 1)     scalar (semi 2)     
error     EOC       error     EOC       
9.61e-04  -         9.61e-04  -         
6.31e-05  3.93      6.31e-05  3.93      
4.09e-06  3.95      4.09e-06  3.95      

mean      3.94      mean      3.94      
----------------------------------------------------------------------------------------------------
(Dict(:l2 => [4.090696282565419 4.090696282565263; 3.9854659838820132 3.985465983898814], :linf => [3.9270697255568128 3.927069726249952; 3.947878553740121 3.9478785702678123]), Dict{Symbol, LinearAlgebra.Transpose{Float64, Matrix{Float64}}}(:l2 => [0.00013318279010717573 0.00013318279010712838; 7.816742843172999e-6 7.816742843171064e-6; 4.934930228156311e-7 4.934930228097621e-7], :linf => [0.0009605782290112996 0.0009605782290100784; 6.314906968718503e-5 6.314906965676492e-5; 4.092013831336061e-6 4.092013782486248e-6]))

This PR has had significant contributions by the LLM Claude.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 23.28358% with 514 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (sc/p4est-view-coupled-enhanced@cdd1181). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/solvers/dgsem_p4est/dg_2d_coupled.jl	0.00%	257 Missing ⚠️
src/meshes/p4est_mesh_view.jl	0.00%	140 Missing ⚠️
src/solvers/dgsem_p4est/containers_coupled.jl	0.00%	44 Missing ⚠️
...discretization/semidiscretization_coupled_p4est.jl	0.00%	43 Missing ⚠️
...dgsem/elixir_advection_coupled_with_amr_mortars.jl	0.00%	20 Missing ⚠️
src/solvers/dgsem_p4est/dg.jl	0.00%	4 Missing ⚠️
src/solvers/dgsem/basis_lobatto_legendre.jl	91.67%	2 Missing ⚠️
src/solvers/dgsem_p4est/dg_2d.jl	0.00%	2 Missing ⚠️
src/solvers/dgsem_p4est/dg_3d_parallel.jl	0.00%	1 Missing ⚠️
src/solvers/dgsem_tree/subcell_finite_volume_O2.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@                        Coverage Diff                        @@
##             sc/p4est-view-coupled-enhanced    #2598   +/-   ##
=================================================================
  Coverage                                  ?   91.87%           
=================================================================
  Files                                     ?      570           
  Lines                                     ?    44734           
  Branches                                  ?        0           
=================================================================
  Hits                                      ?    41095           
  Misses                                    ?     3639           
  Partials                                  ?        0           

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[github-actions]

[JuliaFormatter] _{reported by reviewdog 🐶}

Suggested change

	return integrate_via_indices(func, trixi_backend(_u), _u, mesh, equations, dg, cache,
	return integrate_via_indices(func, trixi_backend(_u), _u, mesh, equations, dg,
	cache,