Characteristic function for Univariate Mixtures

[Thibaut Lamadon]

I would like to suggest adding this method to compute the characteristic function of Univariate Mixtures:

function cf(d::UnivariateMixture,t)
    p = probs(d)
    m = sum(pi * cf(component(d, i),t) for (i, pi) in enumerate(p) if !iszero(pi))
    return m
end

This is a simple copy of the mean function but applied to the characteristic function. So I think it could be added next to it:

Distributions.jl/src/mixtures/mixturemodel.jl

Line 173 in c1705a3

 function mean(d::UnivariateMixture) 

It matches the general cf signature as seen here:

Distributions.jl/src/univariates.jl

Lines 293 to 297 in c1705a3

 """ 
     cf(d::UnivariateDistribution, t) 
  
 Evaluate the characteristic function of distribution `d`. 
 """