The `step` function should have a generic `n_steps > 1` implementation and specialized `n_steps == 1` implementation

[nsiccha]

As in the title and inspired by #485, see e.g.

AdvancedHMC.jl/src/integrator.jl

Lines 216 to 265 in 6bc0c74

	function step(
	lf::DefaultLeapfrog{FT,T},
	h::Hamiltonian,
	z::P,
	n_steps::Int=1;
	fwd::Bool=n_steps > 0, # simulate hamiltonian backward when n_steps < 0
	full_trajectory::Val{FullTraj}=Val(false),
	) where {FT<:AbstractFloat,T<:AbstractScalarOrVec{FT},P<:PhasePoint,FullTraj}
	n_steps = abs(n_steps) # to support `n_steps < 0` cases
	ϵ = fwd ? step_size(lf) : -step_size(lf)
	ϵ = ϵ'
	res = FullTraj ? Vector{P}(undef, n_steps) : nothing
	(; θ, r) = z
	(; value, gradient) = z.ℓπ
	for i in 1:n_steps
	# Tempering
	r = temper(lf, r, (i=i, is_half=true), n_steps)
	# Take a half leapfrog step for momentum variable
	r = r - ϵ / 2 .* gradient
	# Take a full leapfrog step for position variable
	∇r = ∂H∂r(h, r)
	θ = θ + ϵ .* ∇r
	# Take a half leapfrog step for momentum variable
	(; value, gradient) = ∂H∂θ(h, θ)
	r = r - ϵ / 2 .* gradient
	# Tempering
	r = temper(lf, r, (i=i, is_half=false), n_steps)
	# Create a new phase point by caching the logdensity and gradient
	z = phasepoint(h, θ, r; ℓπ=DualValue(value, gradient))
	# Update result
	if !isnothing(res)
	res[i] = z
	end
	if !isfinite(z)
	# Remove undef
	if !isnothing(res)
	resize!(res, i)
	end
	break
	end
	end
	return if FullTraj
	res
	else
	z
	end
	end