Refactor kron_pow

[schillic]

julia> using ReachabilityAnalysis

julia> H = Hyperrectangle(low=[0.9, -0.1], high=[1.1, 0.1])
Hyperrectangle{Float64, Vector{Float64}, Vector{Float64}}([1.0, 0.0], [0.10000000000000009, 0.1])

julia> convert(IntervalBox, ReachabilityAnalysis.kron_pow(H, 2))
[0.81, 1.21001] × [-0.110001, 0.110001] × [-0.110001, 0.110001] × [-0.0100001, 0.0100001]

julia> using DynamicPolynomials

julia> convert(IntervalBox, ReachabilityAnalysis.kron_pow(H, 2))
[0.81, 1.21001] × [-0.110001, 0.110001] × [-0.110001, 0.110001] × [0, 0.0100001]