Fix enstrophy computation for DGMulti

[DanielDoehring]

The original implementation of the enstrophy analysis callback introduced in PR1239 does not work and was not tested.

It is removed in #2140 .

A starting point for a fix would be (note that there are already couple changes to the original version)

function integrate(func::typeof(enstrophy), u,
                   mesh::DGMultiMesh,
                   equations, equations_parabolic::CompressibleNavierStokesDiffusion3D,
                   dg::DGMulti,
                   cache, cache_parabolic; normalize=true)
  gradients_x, gradients_y, gradients_z = cache_parabolic.gradients
  # allocate local storage for gradients.
  # TODO: can we avoid allocating here?
  local_gradient_quadrature_values = ntuple(_ -> copy(cache_parabolic.local_u_values_threaded), 3)
  integral = zero(eltype(eltype(u)))
  for e in eachelement(mesh, dg)
    u_quadrature_values = cache_parabolic.local_u_values_threaded[Threads.threadid()]
    gradient_x_quadrature_values = local_gradient_quadrature_values[1][Threads.threadid()]
    gradient_y_quadrature_values = local_gradient_quadrature_values[2][Threads.threadid()]
    gradient_z_quadrature_values = local_gradient_quadrature_values[3][Threads.threadid()]
    # interpolate to quadrature on each element
    apply_to_each_field(mul_by(dg.basis.Vq), u_quadrature_values, view(u, :, e))
    apply_to_each_field(mul_by(dg.basis.Vq), gradient_x_quadrature_values, view(gradients_x, :, e))
    apply_to_each_field(mul_by(dg.basis.Vq), gradient_y_quadrature_values, view(gradients_y, :, e))
    apply_to_each_field(mul_by(dg.basis.Vq), gradient_z_quadrature_values, view(gradients_z, :, e))
    # integrate over the element
    for i in eachindex(u_quadrature_values)
      gradients_i = SVector(gradient_x_quadrature_values[i],
                            gradient_y_quadrature_values[i],
                            gradient_z_quadrature_values[i])
      integral += mesh.md.wJq[i, e] * func(u_quadrature_values[i], gradients_i, equations_parabolic)
    end
  end
  return integral
end