extract.variance_covariance
return the adequate sub-matrix.
"extract"(struct, what = c("YY", "YZ", "ZZ"), masque_data = c(rep(TRUE, attr(struct, "ntaxa") * attr(struct, "p_dim")), rep(FALSE, (dim(struct)[1] - attr(struct, "ntaxa")) * attr(struct, "p_dim"))))
compute_variance_covariance