IAM_VnkPoisson function

Compute the value V(n,k), needed to caclulate the eppf of a Finite Dirichlet process when the prior on the component-weigts of the mixture is a Dirichlet with parameter gamma (i.e. when unnormailized weights are distributed as Gamma($\gamma$,1) ) when the prior on the number of componet is Shifted Poisson of parameter Lambda. See Section 9.1.1 of the Paper Argiento de Iorio 2019.