[New Copula] Liouville copula class

src/LiouvilleCopula.jl

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+"""
+    LiouvilleCopula{d, T, TG}
+
+Fields: 
+    - \alpha::NTuple{d,T} : the weights for each dimension
+    - G::TG : the generator <: Generator. 
+
+Constructor: 
+
+    LiouvilleCopula(α::Vector{T},G::Generator)
+
+The Liouville copula has a structure that resembles the [`ArchimedeanCopula`](@ref), when you look at it from it's radial-simplex decomposition. 
+
+Recalling that, for C an archimedean copula with generator ``\\phi``, if ``\\mathbf U \\sim C``, then ``U \\equal R \\mathbf S`` for a random vector ``\\mathbf S \\sim`` `Dirichlet(ones(d))`, that is uniformity on the d-variate simplex, and a non-negative random variable ``R`` that is the Williamson d-transform of `\\phi`. 
+
+The Liouville copula has exactly the same expression but using another Dirichlet distribution instead than uniformity over the simplex. 
+
+References: 
+    McNeil, A. J., & Nešlehová, J. (2010). From archimedean to liouville copulas. Journal of Multivariate Analysis, 101(8), 1772-1790.
+"""
+struct LiouvilleCopula{d,TG} <: Copula{d}
+    α::NTuple{d,Int}
+    G::TG
+    function LiouvilleCopula(α::Vector{Int},G::Generator)
+        d = length(α)
+        @assert sum(α) <= max_monotony(G) "The generator you provided is not monotonous enough (the monotony index must be greater than sum(α), and thus this copula does not exists."
+        return new{d, typeof(G)}(G)
+    end
+end
+williamson_dist(C::LiouvilleCopula{d,TG}) where {d,TG} = williamson_dist(C.G,sum(C.α))
+
+function Distributions._rand!(rng::Distributions.AbstractRNG, C, x::AbstractVector{T})
+    # By default, we use the williamson sampling. 
+    r = Distributions.rand(williamson_dist(C))
+    for i in 1:length(C)
+        x[i] = Distributions.rand(rng,Gamma(C.α[i],1))
+        x[i] = x[i] * r
+        x[i] = Distributions.cdf(williamson_dist(C.G,C.α[i]),x[i])
+    end
+    return x
+end
+function _cdf(C::CT,u) where {CT<:LiouvilleCopula}
+    d = length(C)
+
+    sx = sum(Distributions.quantile(williamson_dist(C.G,C.α[i]),u[i]) for i in 1:d)
+    r = zero(eltype(u))
+    for i in CartesianIndices(α)
+        ii = (ij-1 for ij in Tuple(i))
+        sii = sum(ii)
+        fii = prod(factorial.(ii))
+        # This version is really not efficient as derivatives of the generator ϕ⁽ᵏ⁾(C.G, sii, sx) could be pre-computed since many sii will be the same and sx does never change. 
+        r += (-1)^sii / fii * ϕ⁽ᵏ⁾(C.G, sii, sx) * prod(x .^i)
+    end
+    return r
+end