Function euler 

pub fn euler<F>(
    f: F,
    t0: &ExactNum,
    y0: &ExactNum,
    t1: &ExactNum,
    n_steps: usize,
    p: usize,
    rm: RoundingMode,
) -> Option<(ExactNumArray, ExactNumArray)>
where
    F: FnMut(&ExactNum, &ExactNum, usize, RoundingMode) -> ExactNum,

Explicit Euler for y' = f(t, y) on [t0, t1] with n_steps equal steps.

Returns row arrays (t, y) of length n_steps+1. Same rejection as rk4.