Add HREP and VREP convenience union types

[mforets]

You can also add HPolygonOpt to the Union. How about we define the abbreviation Hrep or HRep or HREP as in

const HREP = Union{HPoly, HPolygon, HPolygonOpt}

and use that instead? This interface would assert addconstraint and remove_redundant_constraints.
We could do something similar for VREP = Union{VPolytope, VPolygon}.

Originally posted by @schillic in #2654 (comment)