Write helper function to test if a matrix is diagonal.

[John Ramey]

[John Ramey]

Here is an example, where we test if the Frobenius norm of the difference between the matrix consisting of the diagonal elements of x and x itself is less than some tolerance.

is_diagonal <- function(x, tol = 1e-8) {
  norm(diag(diag(x)) - x, "F") < tol
}