Merge code for overapproximate of TaylorModelReachSet

[schillic]

1. The methods

function _overapproximate(R::TaylorModelReachSet{N}, ::Type{<:Hyperrectangle};
                          Δt::TimeInterval=tspan(R), dom=symBox(dim(R))) where {N}
function _overapproximate(R::TaylorModelReachSet{N}, ::Type{<:Zonotope};
                          Δt::TimeInterval=tspan(R), dom=symBox(dim(R))) where {N}

only differed in the last line (modulo reordering of other lines).

2. The method

function overapproximate(R::TaylorModelReachSet{N}, ::Type{<:Zonotope}, t::AbstractFloat;
                         remove_zero_generators=true) where {N}

was almost identical too; now it calls the common method above instead. Furthermore, a method for time points and Hyperrectangle was not available; now it is.

3. I added a TODO to outsource some code to LazySets. I already added a PR for that here:
   Box approximation of a Taylor model LazySets.jl#3432