Fix bouncing-ball example

[schillic]

There is an error when using GLGM06 with hybrid systems because we do not allow to not intersect with the source invariant anymore (nothing is not a valid parameter anymore because of the restriction to AbstractIntersectionMethod)

ReachabilityAnalysis.jl/src/Hybrid/solve.jl

Line 12 in 1456c39

intersection_method::AbstractIntersectionMethod=HRepIntersection(), # method to take the concrete intersection in discrete transitions

ReachabilityAnalysis.jl/src/Hybrid/solve.jl

Lines 25 to 27 in 1456c39

	ivp_distributed = _distribute(ivp, WLtype; intersection_method=intersection_method,
	check_invariant=check_invariant_initial_states,
	intersect_invariant=intersect_invariant_initial_states)

This means that the else branch below is effectively dead code:

ReachabilityAnalysis.jl/src/Hybrid/solve.jl

Lines 251 to 256 in 1456c39

	if intersection_method != nothing
	STwl = setrep(intersection_method)
	X0 = _overapproximate(X0, STwl)
	else
	STwl = ST
	end

That lead to the bouncing ball failing because X0 is converted to an HPolytope, which the discretization cannot deal with. One problem is that discretize did not print any error because it just uses the no-op default method. I now changed the code to at least print an error message if the discretization failed.

There is a similar error in faq.md (which is why the last code block here is not executed). But I did not investigate if we still have an algorithm to support a ConvexHullArray.