Precision loss in conversion between Taylor model and zonotope of low order

[schillic]

using ReachabilityAnalysis, Plots
Z = Zonotope([0.0, 0], [1.0; 2;;])
TM = overapproximate(Z, TaylorModelReachSet)
Z2 = overapproximate(TM, Zonotope)

julia> Z
Zonotope{Float64, Vector{Float64}, Matrix{Float64}}([0.0, 0.0], [1.0; 2.0;;])

julia> Z2.X
Zonotope{Float64, Vector{Float64}, Matrix{Float64}}([0.0, 0.0], [1.0 1.0; 2.0 2.0])

plot(Z2.X, ls=:dash, lc=:red)
plot!(Z, lc=:blue, lw=2)

This is probably the cause of #623.

[schillic]

The problem is that the conversion from zonotope to Taylor model assumes that the order is at least 1.

ReachabilityAnalysis.jl/src/ReachSets/TaylorModelReachSet.jl

Lines 500 to 505 in 12451d0

	# for each variable i = 1, .., n, compute the linear polynomial that covers
	# the line segment corresponding to the i-th edge of Z
	@inbounds for i in 1:n
	pi = c[i] + sum(view(G, i, :) .* x)
	vTM[i] = TaylorModel1(Taylor1(pi, orderT), zeroI, zeroI, Δtn)
	end