DOCS: add usage of `integrate` for spline
//!
//! ```rust
//! use peroxide::fuga::*;
+//! use std::f64::consts::PI;
//!
//! fn main() {
//! let x = seq(0, 10, 1);
//! cs_akima.integral().polynomial_at(0f64).print();
//! cs_quad.integral().polynomial_at(0f64).print();
//!
+//! println!("============ Integrate from x=0 to x=pi ============");
+//!
+//! cs.integrate((0f64, PI)).print();
+//! cs_akima.integrate((0f64, PI)).print();
+//! cs_quad.integrate((0f64, PI)).print();
+//!
//! // ============ Polynomial at x=0 ============
//! // -0.1523x^3 + 0.9937x
//! // 0.1259x^3 - 0.5127x^2 + 1.2283x
//! // -0.0381x^4 + 0.4969x^2
//! // 0.0315x^4 - 0.1709x^3 + 0.6141x^2
//! // -0.0000x^4 - 0.1289x^3 + 0.6141x^2
+//! // ============ Integrate from x=0 to x=pi ============
+//! // 1.9961861265456702
+//! // 2.0049920614062775
+//! // 2.004327391790717
//! }
//! ```
//!
