Add demo of iterator based topological relations types

[halleysfifthinc]

As we discussed on Zulip, here is a draft PR of my implementation of the topological
relations for HalfEdgeTopologys using the iterator interface. You are welcome to add/use
these however much or little you want, but I have no immediate need for these to be
upstreamed.

Script showing performance benefits of the non-allocating iteration interface:

function vertex_normals_master(msh, fnorms)
    vT = eltype(fnorms)
    vnorms = similar(fnorms, nvertices(msh))
    C = Coboundary{0,2}(topology(msh))

    for i in eachindex(vnorms)
        v = zero(vT)
        for j in C(i)
            v += fnorms[j]
        end
        vnorms[i] = Meshes.unormalize(v)
    end
    return vnorms
end

function vertex_normals_iterable(msh, fnorms)
    vT = eltype(fnorms)
    vnorms = similar(fnorms, nvertices(msh))
    S = Shared{FACE,VERTEX}(msh)

    for i in eachindex(vnorms)
        v = zero(vT)
        for j in S(i)
            v += fnorms[j]
        end
        vnorms[i] = Meshes.unormalize(v)
    end
    return vnorms
end

sphere = topoconvert(HalfEdgeTopology, simplexify(Sphere(Point(0,0,0), 1)))
fnorm = normal.(elements(sphere))

julia> @benchmark vertex_normals_master(sphere, fnorm)
BenchmarkTools.Trial: 2521 samples with 1 evaluation per sample.
 Range (min … max):  1.664 ms …  18.377 ms  ┊ GC (min … max): 0.00% … 85.70%
 Time  (median):     1.763 ms               ┊ GC (median):    0.00%
 Time  (mean ± σ):   1.975 ms ± 912.216 μs  ┊ GC (mean ± σ):  7.22% ± 11.68%

  ██▅▃▃▂▂▁                                                    ▁
  █████████▇█▆▃▅▃▃▁▃▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▄▅▆▆▆▆▆▇▆ █
  1.66 ms      Histogram: log(frequency) by time      6.67 ms <

 Memory estimate: 1.89 MiB, allocs estimate: 61831.

julia> @benchmark vertex_normals_iterable(sphere, fnorm)
BenchmarkTools.Trial: 10000 samples with 1 evaluation per sample.
 Range (min … max):  169.017 μs …  6.926 ms  ┊ GC (min … max): 0.00% … 95.40%
 Time  (median):     181.195 μs              ┊ GC (median):    0.00%
 Time  (mean ± σ):   191.566 μs ± 89.738 μs  ┊ GC (mean ± σ):  1.33% ±  3.32%

  ▂▆█▅▃▄▅▅▅▄▅▅▃▂▂▁▁▃▃▃▂▂▃▃▂▃▄▅▃▂▂▂▁▁▂▂▁     ▁▁ ▁▁▁▁▁▁▁         ▂
  █████████████████████████████████████▇▇▆█████████████▆▆▆▅▆▅▅ █
  169 μs        Histogram: log(frequency) by time       248 μs <

 Memory estimate: 59.88 KiB, allocs estimate: 3.