This function extracts the full covariance matrices from a mixed/hierarchical
linear model fit using lme.
extract_design(b)A list of matrices is returned.
D contains the covariance matrix of the random effects.
V contains the covariance matrix of the response.
X contains the fixed-effect model matrix.
Z contains the random-effect model matrix.
a fitted model object of class lme.
Adam Loy loyad01@gmail.com
This method has been adapted from the method
mgcv::extract.lme.cov in the mgcv package, written by Simon
N. Wood simon.wood@r-project.org.