Option to concretize discretization sets

[schillic]

Several discretization methods apply the set operations lazily and return these lazy sets. But often one wants to apply them concretely because the iteration becomes expensive otherwise.

I checked two cases, but there may be more.

• Forward has an option for Ω0 but not for V (called Ud here), which I believe is a bug:

ReachabilityAnalysis.jl/src/Discretization/ForwardModule.jl

Lines 144 to 148 in 4d1ef9c

	Ud = δ * U ⊕ Eψ0
	In = IdentityMultiple(one(eltype(A)), size(A, 1))
	Ω0 = ConvexHull(X0, Φ * X0 ⊕ Ud ⊕ E⁺)
	Ω0 = _apply_setops(Ω0, alg.setops)

• FirstOrder does not offer an option at all, which should be changed:

ReachabilityAnalysis.jl/src/Discretization/FirstOrderModule.jl

Lines 98 to 102 in 4d1ef9c

	Ω0 = ConvexHull(X0, Xδ + δ * U + BallInf(zeros(n), α))
	# discretize inputs
	β = factor * norm_U_over_A
	V = δ * U + BallInf(zeros(n), β)