Remove witness field from properties

[schillic]

We always ask for a witness in the properties and store them. However, in general the witness production is more expensive, and we never used the witness so far. Furthermore, the property types are currently mutable for this reason, and witnesses cannot be stored with Conjunction etc. (see #474).

I propose to instead add an optional argument witness::Bool=false to the check function, and then optionally return the witness immediately to the caller.

[schillic]

Comment from here by @mforets:

Tested independently:

julia> using LazySets; R = rand(Hyperrectangle, dim=100); H = rand(HalfSpace, dim=100);

julia> @btime is_intersection_empty($R, $H, false)
  272.878 ns (3 allocations: 1.77 KiB)
false

julia> @btime is_intersection_empty($R, $H, true);
  771.026 ns (5 allocations: 1.81 KiB)

julia> R = rand(Hyperrectangle, dim=2); H = rand(HalfSpace, dim=2);

julia> @btime is_intersection_empty($R, $H, false);
  152.680 ns (3 allocations: 208 bytes)

julia> @btime is_intersection_empty($R, $H, true);
  589.767 ns (6 allocations: 336 bytes)