getV: Determine V-Matrix for a 'VCA' Object.
Description
Determine the estimated variance-covariance matrix of observations $y$.
Value
(VCA) object with additional elements in the 'Matrices' element, including matrix $V$.
Details
A linear mixed model can be written as $y = Xb + Zg + e$, where $y$ is the column
vector of observations, $X$ and $Z$ are design matrices assigning fixed ($b$),
respectively, random ($g$) effects to observations, and $e$ is the column vector of
residual errors.
The variance-covariance matrix of $y$ is equal to $Var(y) = ZGZ' + R$, where $R$
is the variance-covariance matrix of $e$ and $G$ is the variance-covariance matrix of $g$.
Here, $G$ is assumed to be a diagonal matrix, i.e. all random effects $g$ are mutually independent
(uncorrelated).