Add discrete post for GLGM06

[schillic]

This is an example how to add discrete post functions for the linear algorithms.
solve is not implemented yet.

function exponential_1d(; invariant=Universe(1))
  s = @system(x' = -x, x ∈ invariant)
  x0 = Interval(0.4, 0.5)
  prob = InitialValueProblem(s, x0)
  tspan = (0.0, 1.0)
  return prob, tspan
end
ivp, _ = exponential_1d()
alg = GLGM06(; δ=0.01)

sol_c = solve(ivp; T=5.0, alg=alg);

ivp_norm = ReachabilityAnalysis._normalize(ivp)
ivp_discr = discretize(ivp_norm, alg.δ, alg.approx_model)
NSTEPS = 500

fp_d = ReachabilityAnalysis.post(alg, ivp_discr, NSTEPS);

At the end, flowpipe(sol_c) == fp_d would hold, but == is not defined for flowpipes.