Fold IDs of sibship grandparents into a 2 x nInd/2 x 2 array, as they are stored in Fortran, and then stretch this into a vector that can be passed to Fortran and easily be transformed back into said 3D array.
FoldSibGPs(PedNum, Ng, Nd)
An integer vector, with missing values as 0.
pedigree, ids replaced by numbers, dummies negative.
no. genotyped indivs.
length 2 vector, no. female & male dummies.