Dimension error for default Polyhedra backend in >= v0.7

[schillic]

With #831 we introduced a misuse in the default backend of Polyhedra: Here we should not pass 2 but dim(P) (and the same a few lines below). The problem is that we do not have P here...

[mforets]

yes, i hardcoded 2 in default_polyhedra_backend because the default_library for d > 1 does not depend on d, there is just one DefaultLibrary{_default_type(T)}(), see this function.

presumably the default interval library is specialized for interval computations, and i would expect that the general library also works for d=1 (but i havent' checked).

if we want we can still update our functions passing P as well, eg.

function convex_hull(P1::HPoly{N},
                     P2::HPoly{N};
                     backend=default_polyhedra_backend(P1, N)) where {N}
    @assert isdefined(Main, :Polyhedra) "the function `convex_hull` needs " *
                                        "the package 'Polyhedra' to be loaded"
    Pch = convexhull(polyhedron(P1, backend), polyhedron(P2, backend))
    return convert(typeof(P1), Pch)
end

...

    function default_polyhedra_backend(P, N::Type{<:AbstractFloat})
        return default_library(LazySets.dim(P), Float64)
    end

[schillic]

i would expect that the general library also works for d=1 (but i havent' checked).

There are problems with 1-dimensional cases, see #843.