Generalize exponential backend in symmetric_interval_hull methods

[schillic]

The Approximations module only supports Expokit as exponential backend:

LazySets.jl/src/Approximations/symmetric_interval_hull.jl

Lines 121 to 131 in 91fdb61

	function symmetric_interval_hull(E::ExponentialMap{N, <:AbstractSingleton}) where {N}
	require(@__MODULE__, :Expokit; fun_name="symmetric_interval_hull")
	v = expmv(one(N), E.spmexp.M, element(E.X))
	c = zeros(N, dim(E))
	r = abs.(v)
	return Hyperrectangle(c, r)
	end
	function symmetric_interval_hull(E::ExponentialMap{N, <:AbstractHyperrectangle}) where {N}
	require(@__MODULE__, :Expokit; fun_name="symmetric_interval_hull")

Instead it should use the code from the main LazySets module:

LazySets.jl/src/Utils/matrix_exponential.jl

Line 14 in 91fdb61

function get_exponential_backend()