Function for applying R inverse to u in REML EM algorithm
Rinv.u(G, AtG, GtAAtG, GtAu, u, tau, sigma)Matrix of genotype markers (size n x L)
AtG from precomputation
GtAAtG from precomputation
GtAu from precomputation
u=Aty from REML EM
Variance component from G main effect
Variance component from model noise epsilon
Vector resulting from left multiplication of Rinv with input vector u