mnix_suff: Sufficient statistics for the mNIX distribution.
In mlysy/losmix: Inference for Gaussian Location-Scale Mixed-Effects Models

Sufficient statistics for the mNIX distribution.

1	mnix_suff(id, y, X)

`id`	Subject identifiers. A vector of length `n` consisting of `nsub <= n` distinct elements. If missing default to `rep(1, n)`, i.e., only one subject.
`y`	Response vector for all subjects. A vector of length `n`.
`X`	Covariate matrix from all subjects. A matrix of size `n x p`.

A list with elements:

yy: A vector of length nsub consisting of t(y) y for each subject.
Xy: A matrix of size p x nsub consisting of t(X) y for each subject.
XX: A matrix of size p x nsub^2 consisting of t(X) X for each subject.