Set up an unstructured (UNSTR) correlation term in brms. The function does
not evaluate its arguments -- it exists purely to help set up a model with
UNSTR terms.

## Arguments

- time
An optional time variable specifying the time ordering
of the observations. By default, the existing order of the observations
in the data is used.

- gr
An optional grouping variable. If specified, the correlation
structure is assumed to apply only to observations within the same grouping
level.

## Value

An object of class `'unstr_term'`

, which is a list
of arguments to be interpreted by the formula
parsing functions of brms.

## Examples

```
if (FALSE) {
# add an unstructured correlation matrix for visits within the same patient
fit <- brm(count ~ Trt + unstr(visit, patient), data = epilepsy)
summary(fit)
}
```