Calculate the correlation of the design. The function gets the design from
the design object before passing it to `cor`

from stats.
This is a wrapper around `cor`

.

## Arguments

- x
A model object of class 'spdesign'

- ...
Additional parameters passed to the function

## Value

A matrix with correlations

## Details

Note that when your design includes constants, the function will print a
warning because the standard deviation of a constant is 0.