This function convert parameters from a Normal-Inverse-Chi-Squared
(\((\mu,\sigma^2) \sim NIX({\sf mean}=\mu,{\sf effective sample size}=\kappa,{\sf degrees of freedom}=\nu,{\sf variance}=\sigma^2/\kappa)\))
distribution to a Normal-Inverse-Gamma
(\((\mu,\sigma^2) \sim NIG({\sf mean}=m,{\sf variance}=V \times \sigma^2,{\sf shape}=a,{\sf rate}=b)\))
distribution.