new latexify setup to support colored trees

[ranocha]

This allows us to create LaTeX code rendered as follows:

Code to reproduce:

julia> using BSeries, Latexify

julia> ark = let # Störmer-Verlet method
       As = [
           [0 0; 1//2 1//2],
           [1//2 0; 1//2 0],
       ]
       bs = [
           [1 // 2, 1 // 2],
           [1 // 2, 1 // 2],
       ]
       AdditiveRungeKuttaMethod(As, bs)
       end

julia> series = bseries(ark, 3)

julia> latexify(series; cdot = false)

Compared to the previous version, the LaTeX code of plain (single-colored) rooted trees is a bit more verbose since it includes an additional . after each opening bracket [, e.g.,

julia> using RootedTrees

julia> t = rootedtree([1, 2, 2])
RootedTree{Int64}: [1, 2, 2]

julia> RootedTrees.latexify(t) |> println
\rootedtree[.[.][.]]