format_data: Format data for 'TMB' calculations.
In mlysy/losmix: Inference for Gaussian Location-Scale Mixed-Effects Models

Format data for TMB calculations.

1	format_data(y, X, id)

`y`	Response vector for all subjects. A vector of length `n`.
`X`	Covariate matrix from all subjects. A matrix of size `n x p`.
`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.

A list with the following elements:

y: The response vector y, reordered such that all observations for a given subject are consecutive, and converted to an n x 1 column matrix.
Xtr: The transpose of the covariate matrix, reordered the same way as y, having size p x n.
iStart: An integer vector of length nsub, specifying the starting index of the observations for each subject, using C++ indexing (i.e., starting at zero).
nObs: An integer vector of length nsub, specifying the number of observations per subject.