nix2_update: Conjugate updating of Normal-Chi^-2 parameters
In kleinschmidt/beliefupdatr: Belief updating for phonetic adaptation

1 2	nix2_update(p = list(nu = 0, kappa = 0, mu = 0, sigma2 = 0), x = NA, n = length(x), xbar = mean(x), ss = s2 * (n - 1), s2 = var(x))

`p`	named list of distribution parameters (mu, sigma2, nu, and kappa)
`x`	vector of observations to update with (optional if summary stats are passed)
`n`	number of observations (overrides x)
`xbar`	observed mean (overrides x)
`ss`	observed sum of squares (overrides x and s2)
`s2`	observed sample variance (ss / (n-1); overrides x)