Fix ODE doc test
//!
//! ```rust
//! extern crate peroxide;
-//! use peroxide::fuga::{Real, State, BoundaryCondition};
+//! use peroxide::fuga::{Real, State, BoundaryCondition, Environment};
//!
//! pub trait ODE<E: Environment> {
//! type Records;
//! ```rust
//! extern crate peroxide;
//! use std::collections::HashMap;
-//! use peroxide::fuga::{State, ExMethod, BoundaryCondition, ODEOptions};
+//! use peroxide::fuga::{State, ExMethod, BoundaryCondition, ODEOptions, Environment};
//!
//! #[derive(Clone)]
//! pub struct ExplicitODE<E: Environment> {
