an optional character string with a description of
the varFunc class. Default depends on the method function:
"Combination of variance functions" for varComb,
"Constant plus power of covariate" for varConstPower,
"Exponential of variance covariate" for varExp,
"Different standard deviations per stratum" for varIdent,
"Power of variance covariate" for varPower, and
data.class(object) for varFunc.