_compute_step_acc: compute Pth root in step computation using Rationals instead of floats

This makes working with units more exact, since the powers of Unitful
units are always rational. Without this, we get powers such as
74981274124 / 1249141204412 for units.

Author: SeSodesa

src/methods.jl

@@ -399,7 +399,7 @@ function _compute_step_acc(
     # Set the step size by minimising an upper bound on the error of the estimate.
     C₁ = f_error * m.f_error_mult * m.factor
     C₂ = ∇f_magnitude * m.∇f_magnitude_mult
-    step = (Q / (P - Q) * (C₁ / C₂))^(1 / P)
+    step = (Q / (P - Q) * (C₁ / C₂))^(1 // P)
     # Estimate the accuracy of the method.
     acc = C₁ * step^(-Q) + C₂ * step^(P - Q)
     return step, acc