fit.submodel: Compute the fit of a submodel
In hsstan: Hierarchical Shrinkage Stan Models for Biomarker Selection

Compute the fit of a submodel

1	fit.submodel(x, sigma2, mu, chosen, xt, yt, logistic)

`x`	Design matrix.
`sigma2`	Residual variance (1 for logistic regression).
`mu`	Matrix of fitted values for the full model.
`chosen`	Vector of indices of the columns of `x` in the current submodel.
`xt`	Design matrix for test data.
`yt`	Outcome variable for test data.
`logistic`	Set to `TRUE` for a logistic regression model.