Add `sum` to `VecOps`
//! * `div(&self, other: Vec<f64>) -> Vec<f64>`
//! * `dot(&self, other: Vec<f64>) -> f64`
//! * `norm(&self) -> f64`
+//! * `sum(&self) -> f64`
//!
//! ```rust
//! extern crate peroxide;
//! a.div(&b).print();
//! a.dot(&b).print();
//! a.norm().print();
+//! a.sum().print();
//!
//! // [5, 5, 5, 5]
//! // [-3, -1, 1, 3]
//! // [0.25, 0.6667, 1.5, 4]
//! // 20
//! // 5.477225575051661 // sqrt(30)
+//! // 10
//! }
//! ```
//!
