Add saturating_add for PID final summation

pid/src/lib.rs

@@ -153,7 +153,7 @@ impl<T: SaturatingAdd + SaturatingSub + Scaled + Copy> Pid<T> {
         let p = error.saturating_mul_unscaled(self.k_p);
         let i = self.integrated_error.saturating_mul_unscaled(self.k_i);
         let d = deriv_error.saturating_mul_unscaled(self.k_d);
-        (p + i + d).rescale()
+        p.saturating_add(&i).saturating_add(&d).rescale()
     }
 
     pub fn command(&mut self, target: T, measured: T) -> T {