function createCovarianceMatrixFromUpperTriangle(array) {
  const covarianceMatrix = new Matrix3(
    array[0],
    array[1],
    array[3],
    array[1],
    array[2],
    array[4],
    array[3],
    array[4],
    array[5],
  );
  return covarianceMatrix;
}
Creates a Matrix3 that describes a covariance matrix (which is
symmetric) from the array containing the upper triangle, in
column-major order.

@param {number[]} array The input array
@returns {Matrix3} The Matrix3