A generalized least squares structure is a list of model components
representing different sets of parameters in the linear
model. A glsStruct may contain corStruct and
varFunc objects. NULL arguments are not included in the
an optional corStruct object, representing a
correlation structure. Default is NULL.
an optional varFunc object, representing a
variance function structure. Default is NULL.
a list of model variance-covariance components determining the
parameters to be estimated for the associated linear model.