Make HPolygonOpt a subtype of HPolygon

[schillic]

The only type union not affected by #19 exists in Polgyon.jl to handle HPolygons and HPolygonOpts simultaneously.

We should probably just make HPolygonOpt inherit from HPolygon.
This makes sense both from the code and from the conceptual side to me.
I am, however, not sure if Julia finds the correct function if one argument is an array of a subtype; I guess it does, but this has to be tested.

@mforets: What do you think?

[mforets]

hmm.. i dunno... otherwise we can group them all into a newabstract type LazyPolygon <: LazySet end in the sense that HPolygon <: LazyPolygon, VPolygon <: LazyPolygon and HPolygonOpt <: LazyPolygon.

[schillic]

My motivation was that all functions that handle HPolygon can automatically handle HPolygonOpt.
There is no structural difference.

This is not true for VPolygon.

[mforets]

that makes sense. ok, then let me do that. today i'll be working on this one and on #13. (i'll put in the same PR).

[schillic]

Sorry, subtyping is not allowed in Julia (source).