Discuss new `L2GTransformer` constructor

[pjaap]

See @j-fu comment in #100:

I would agree. But if there is time: I thought a bit more on this. Adding a type stable method

function L2GTransformer(
        EG::Union{Type{<:Tetrahedron3D}, Type{<:Parallelepiped3D}},
        grid::ExtendableGrid{Tv, Ti},
        AT::Type{<:AssemblyType},
        CT::Type{<:AbstractCoordinateSystem}
    ) where {Tv, Ti}
    A = zeros(Tv, size(grid[Coordinates], 1), dim_element(EG))
    b = zeros(Tv, size(grid[Coordinates], 1))
    return L2GTransformer{Tv, Ti, EG, CT}(
        0,
        false,
        grid[Coordinates],
        grid[GridComponentNodes4AssemblyType(AT)],
        grid[GridComponentVolumes4AssemblyType(AT)],
        A,
        b,
        zeros(Tv, 3, 3),
        0
    )
end

could help to solve this. However one would have to do more at the call site as well.

Originally posted by @j-fu in #100 (comment)

[Da-Be-Ru]

@chmerdon examined the problem a little more closely and noticed that the update_trafo! allocation does show up, but only on a sufficiently large grid. Specifically, the allocation starts showing up as soon as $\mathtt{icell}\geq512$ on a grid_unitsquare.

We're not quite sure yet why this happens exactly.