Allow different type for LazyRow index

Having the possibility to have a different type for the LazyRow index other than I

struct LazyRow{T, N, C, Ic, Ii}
    columns::StructArray{T, N, C, Ic} # a `Columns` object
    index::Ii
end

allows me to define a StructArray of points and a StructArray of triangles, with indexes being of type StaticVectors (a triangle always contains 3 points):

struct Point{T}
    position::SVector{3,T}
    force::SVector{3,T}
end

struct Triangle{Tp}
    points::Tp
end

points = StructArray([Point{Float64}(rand(3), zeros(3)) for _ in 1:10])

tris = StructArray([Triangle(LazyRow(points, @SVector [rand(1:10), rand(1:10), rand(1:10)])) for _ in 1:5])

I can now update the point forces in the triangles this way, which is really helpful in my case:

for t in tris
    pos = t.points.position
    f1 = sum.(pos)
    f2 = tan.(f1)
    f3 = sin.(f1)
    f = Vec3(f1, f2, f3)
    t.points.force += f
end

Does that make sense? Would that make some thing break somewhere?

Thanks

Start by locating the LazyRow definition and tracing how its index type is used in construction, indexing, and mutation. Check whether the proposed StaticVector index works without changing existing LazyRow behavior; done means distinct index types are supported and the point/triangle example works without regressions.