Fix error of test
//! use peroxide::*;
//!
//! fn main() {
-//! let mut m = DataFrame::with_header(vec!["x", "y", "z"]);
+//! #[cfg(feature = "dataframe")]
+//! {
+//! let mut m = DataFrame::with_header(vec!["x", "y"]);
//! m["x"] = c!(1,2,3);
//! m["y"] = c!(3,2,1);
//!
//! // r[0] 1.0000 -1.0000
//! // r[1] -1.0000 1.0000
//! }
+//! }
//! ```
use structure::matrix::*;
