Add benchmark scripts from ExclusiveTopology to benchmark folder

[KnutAM]

Can you dump the benchmark code in the benchmark folder (here or separate PR)?

Originally posted by @termi-official in #974 (review)

Would be good to do, relevant scripts are here for ref. I'll try to find time to look into that, unless someone does it before me.

using Ferrite, BenchmarkTools
function sum_indices(nbh::AbstractMatrix)
    v = 0
    for nbs in nbh
        for n in nbs
            v += n[1] + n[2]
        end
    end
    return v
end

function sum_indices(top::ExclusiveTopology)
    v  = sum_indices(top.vertex_vertex_neighbor)
    v += sum_indices(top.edge_edge_neighbor)
    v += sum_indices(top.face_face_neighbor)
    return v
end
function sum_skeleton_facet_neighbors(top, grid)
    skeleton = Ferrite.facetskeleton(top, grid)
    v = 0
    for facetidx in skeleton
        v += sum(n[1] + n[2] for n in getneighborhood(top, grid, facetidx); init = 0)
    end
    return v
end

grid = generate_grid(Hexahedron, (10, 10, 10));
top = ExclusiveTopology(grid); # Compilation

@btime sum_indices($top); 
# Master: 55.700 μs (0 allocations: 0 bytes)
#     PR: 36.000 μs (0 allocations: 0 bytes)
Ferrite.facetskeleton(top, grid); # Compilation

grid = generate_grid(Hexahedron, (100, 100, 100));
@time top = ExclusiveTopology(grid);
# Master: 27.150434 seconds (64.58 M allocations: 7.195 GiB, 72.43% gc time)
#     PR:  3.799651 seconds (45 allocations: 1.895 GiB, 13.84% gc time)
@time Ferrite.facetskeleton(top, grid);
# Master: 0.143506 seconds (2 allocations: 46.234 MiB)
#     PR: 0.032492 seconds (2 allocations: 46.234 MiB)
@btime sum_skeleton_facet_neighbors($top, $grid);
# Master: 59.683 ms (0 allocations: 0 bytes)
#     PR: 21.887 ms (0 allocations: 0 bytes)