Powerspectrum estimator add threshold

[elavallee]

Please review my change to add a threshold to the PowerSpectrum estimator. The optional threshold is named: $\delta$. I updated the docstring, and I was able to pass all the tests locally. However, I did not add any new tests, which is more challenging. For an example of how to use the new feature, please see the code below.
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using DynamicalSystems, ComplexityMeasures

N1, N2, a = 101, 100001, 10

for N in (N1, N2)
    println("N: $(N)")
    t = LinRange(0, 2*a*π, N)
    x = sin.(t) # periodic
    y = sin.(t .+ cos.(t/0.5)) # periodic, complex spectrum
    z = sin.(rand(1:15, N) ./ rand(1:10, N)) # random
    w, t = trajectory(Systems.lorenz(), N÷10; Δt = 0.1, Ttr = 100) # chaotic
    w    = w[:, 1]

    for q in (x, y, z, w)
        p, ~ = probabilities_and_outcomes(PowerSpectrum(100.0), q)
        h = entropy(p)
        println("entropy: $(h).")
    end
end

[Datseris]

some simplifying and performance comments

[Datseris]

Suggested change

δ = getfield.(Ref(P), (:δ))

[Datseris]

Suggested change

	amp_squared[0.0 .< amp_squared .< δ] .= 0.0
	if P.δ > 0
	amp_squared[amp_squared .< P.δ] .= 0.0
	end

[Datseris]

Thanks for the PR @elavallee , in addition please increment the minor version (middle number) at Project.toml file by 1 and add an entry about this change in the CHANGELOG.md file!