This function computes the negative predicted log-likelihood from a DGP emulator with a likelihood layer.

## Arguments

- object
an instance of the

`dgp`

class and it should be produced by`dgp()`

with one of the following two settings:- x
a matrix where each row is an input testing data point and each column is an input dimension.

- y
a matrix with only one column where each row is a scalar-valued testing output data point.

## Value

An updated `object`

is returned with an additional slot named `NLL`

that contains two elements.
The first one, named `meanNLL`

, is a scalar that gives the average negative predicted log-likelihood
across all testing data points. The second one, named `allNLL`

, is a vector that gives the negative predicted
log-likelihood for each testing data point.

## Details

See further examples and tutorials at https://mingdeyu.github.io/dgpsi-R/.