Remove doubled saving of subcell limiting factors

[bennibolm]

Before we had alpha, alpha1 and alpha2 in ContainerSubcellLimiterIDP2D.
In alpha, we saved the node-wise limiting factor. In alpha1 and alpha2, basically, we only saved the maximum limiting factor between two nodes

alpha1[i, j, element] = max(alpha[i - 1, j, element], alpha[i, j, element])
alpha2[i, j, element] = max(alpha[i, j - 1, element], alpha[i, j, element])

, which was used only once afterward.
This was convenient since we didn't need any if clause later in the correction stage.
In this PR, I removed alpha1 and alpha2 and compute the maximum when actually needed.

Moreover, to have the nice flux-differencing update in the correction stage

u[v, i, j, element] += dt * inverse_jacobian *
                                       (inverse_weights[i] *
                                        (alpha_flux1_ip1[v] - alpha_flux1[v]) +
                                        inverse_weights[j] *
                                        (alpha_flux2_jp1[v] - alpha_flux2[v]))

we calculated e.g.

alpha_flux1 = (1 - alpha1[i, j, element]) *
                          get_node_vars(antidiffusive_flux1_R, equations, dg,
                                        i, j, element)

for every node, although antidiffusive_flux1 was zero at the element interfaces (i=1 or i=nnodes+1).

To avoid unnecessary access to and further computation with zero entries, we directly use zero vectors in those cases now.
With that, we can keep the nice flux-differencing update from above.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.79%. Comparing base (b15c6ea) to head (f85aaae).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2588   +/-   ##
=======================================
  Coverage   96.79%   96.79%           
=======================================
  Files         525      525           
  Lines       42708    42699    -9     
=======================================
- Hits        41337    41329    -8     
+ Misses       1371     1370    -1     

Flag	Coverage Δ
unittests	96.79% <100.00%> (+<0.01%)	⬆️

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[bennibolm]

I just noticed that it is not necessary to set the antidiffusive flux at the interfaces to zero in each rhs! call as it is done here.

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

Lines 685 to 693 in e842dd2

	antidiffusive_flux1_L[:, 1, :, element] .= zero(eltype(antidiffusive_flux1_L))
	antidiffusive_flux1_L[:, nnodes(dg) + 1, :, element] .= zero(eltype(antidiffusive_flux1_L))
	antidiffusive_flux1_R[:, 1, :, element] .= zero(eltype(antidiffusive_flux1_R))
	antidiffusive_flux1_R[:, nnodes(dg) + 1, :, element] .= zero(eltype(antidiffusive_flux1_R))
	antidiffusive_flux2_L[:, :, 1, element] .= zero(eltype(antidiffusive_flux2_L))
	antidiffusive_flux2_L[:, :, nnodes(dg) + 1, element] .= zero(eltype(antidiffusive_flux2_L))
	antidiffusive_flux2_R[:, :, 1, element] .= zero(eltype(antidiffusive_flux2_R))
	antidiffusive_flux2_R[:, :, nnodes(dg) + 1, element] .= zero(eltype(antidiffusive_flux2_R))

Note: Continued in #2589

[bennibolm]

@DanielDoehring Could you please take a look at this?

[DanielDoehring]

Looks good to me!