Internal function to compute unconditional turning bands simulations
gsi.TurningBands(X, vgram, nBands, nsim = NULL)
matrix of coordinates of locations where realisations are desired
covariogram model, of format "gmCgram"
number of bands to use
number of realisations to return
an array with (npoint, nvar, nsim)-elements, being npoint=nrow(X) and nvar = nr of variables in vgram