Updated the way that steps are handled

src/integrator/mod.rs

@@ -44,7 +44,7 @@ mod tests {
         let y0 = Vector3::new(1.0, 1.0, 1.0);
         let mut ode = ODE::new(&derivative, 0.0, 4.0, y0, ());
 
-        let dp45 = DormandPrince45::new(1e-12_f64, 1e-4_f64);
+        let dp45 = DormandPrince45::new().a_tol(1e-12).r_tol(1e-4);
 
         // Test that y'(t) = y(t) solves to y(t) = e^t for rkf54
         // and also that the error seems reasonable