LazyDiscretePost requires Polyhedra package

[schillic]

(reported by @mforets)

# intersect with target invariant
A⌜R⋂G⌟⋂I = Intersection(target_invariant, A⌜R⋂G⌟)
if isempty(A⌜R⋂G⌟⋂I) ## << requires polyhedra ..... why?
    continue
end

[schillic]

The problem is the application of a non-invertible linear map (thanks @mforets for investigating).

[schillic]

I suggest we add an option to overapproximate the set whenever the matrix is not invertible (and this option should be the default).