If object includes a varStruct component, the inverse of
the standard deviations of the variance function structure represented
by the corresponding varFunc object are returned; else, a
vector of ones of length equal to the number of observations in the
data frame used to fit the associated linear model is returned.
# S3 method for glsStruct
varWeights(object)if object includes a varStruct component, a vector with
the corresponding variance weights; else, or a vector of ones.
Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models in S and S-PLUS", Springer.