Cache box and zonotope oa in TMJets

[mforets]

i'm thinking about computing the zonotopic oa by default, and add a plot recipe for taylor model flowpipes that uses it.

• note that if an invariant is passed, we already compute it inside _is_intersection_empty(Ri, X) if intersection_method is a ZonotopeEnclosure() (the default)

• overapproximate(<taylor model flowpipe>, ::Zonotope) should check if that approx has been computed.

• this change would be aligned to making the library more user-friendly: one gets a first plot for a nonlinear ODE without actually knowing about overapproximations, zonotopes, etc.

---

this can be done by returning a tuple,

function _is_intersection_empty(X::AbstractReachSet, Y::LazySet, ::ZonotopeEnclosure)
    Z = overapproximate(X, Zonotope)
    return (Z, _is_intersection_empty(set(Z), Y))
end