at(Exp).grm(Geno|Fld) expands to a set of model terms, 1 for each Experiment
with a reduced GRM file containing just those Genotypes present in the relevant field;
where Exp is nested in Fld.
residual sat(Exp !VPGROUP Fld).ar1(Row).ar1(Column) applies constrants so that the variance parameters estimated are the same for Experiments in the same Fld.
Simple Modeling of residual variance through a log gamma linear model by fitting
the squared residual as a second trait dev(wt) in a bivariate model
Added !ASUV qualifier for when different weights are required for each trait.