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

## Arguments

- M
Known correlation/covariance matrix of the response variable.
If a vector is passed, it will be used as diagonal entries
(variances) and correlations/covariances will be set to zero.
The actual covariance matrix used in the likelihood is obtained
by multiplying `M`

by the square of the residual standard
deviation parameter `sigma`

estimated as part of the model.

## Value

An object of class `'fcor_term'`

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

## Examples

```
if (FALSE) {
dat <- data.frame(y = rnorm(3))
V <- cbind(c(0.5, 0.3, 0.2), c(0.3, 1, 0.1), c(0.2, 0.1, 0.2))
fit <- brm(y ~ 1 + fcor(V), data = dat, data2 = list(V = V))
}
```