#121 - Add undef and revise polytope/polygon constructors

[schillic]

See #121.

• add undef as commented here and here
• revise initialization of polytope/polygon constructors from empty input; also fix the constructor for VPolytope
• add constructors for VPolygon
• revise initialization of array set type constructors from empty input

[schillic]

Actually, there are more places:

AbstractHPolygon.jl:    points = Vector{Vector{N}}(n)
AbstractPolytope.jl:    new_vlist = Vector{Vector{N}}(length(vlist))
CartesianProduct.jl:    arr = Vector{LazySet{N}}(0)
convert.jl:    constraints_list = Vector{LinearConstraint{N}}(3)
ConvexHull.jl:    a = Vector{LazySet{N}}(0)
Intersection.jl:    arr = Vector{LazySet{N}}(0)
MinkowskiSum.jl:    arr = Vector{LazySet{N}}(0)
MinkowskiSum.jl:    arr = Vector{LazySet{N}}(0)
VPolygon.jl:        constraints_list = Vector{LinearConstraint{N}}(0)

[schillic]

I found a last one in to_N.jl: Matrix{N}(k) is of course also affected. I rebased.