This function takes the entire covariance matrix and creates a matrix of covariances based on the vecchia approximatino object
getMatCov(V, covariances, factor = FALSE)
matrix of size n x (m+1) with only those elements that are used by the incomplete Cholesky decomposition
the object returned by vecchia_specify
The full covariance matrix or a covariance function
True if we are passing a factor of a matrix