This function is a constructor for the varFixed class,
representing a variance function with fixed variances. Letting \(v\)
denote the variance covariate defined in value, the variance
function \(\sigma^2(v)\) for this class is
\(\sigma^2(v)=|v|\). The variance covariate \(v\) is
evaluated once at initialization and remains fixed thereafter. No
coefficients are required to represent this variance function.
a one-sided formula of the form ~ v specifying a
variance covariate v. Grouping factors are ignored.
a varFixed object representing a fixed variance function
structure, also inheriting from class varFunc.
Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models
in S and S-PLUS", Springer.