Add Lotka-Volterra model

[mforets]

https://es.wikipedia.org/wiki/Ecuaciones_Lotka%E2%80%93Volterra

[mforets]

An animation of the flowpipe:

This animation was done with the following code:

plot(Xo, color=:orange, lab="")

anim = @animate for (i, x) in enumerate(sol.Xk)
    segmento = [set(sol.Xk[k]) for k in 1:i]
    plot!(segmento, xlim=(1.0, 6.0), ylim=(0.0, 3.5), xlab=L"x", ylab=L"y",
          lw=0.2, color=:lightblue, lab="", dpi=200)
    plot!(Xo, color=:orange, lab="")
end

gif(anim, "LotkaVolterra.gif", fps=15)

cc: @SebastianGuadalupe