[wpimath] Clean up NumericalIntegration and add Discretization tests (#3489)

* Rename Butcher tableau sections in NumericalIntegration such that
  top-left is c, top-right is A, and bottom-right is b
* Move edu.wpi.first.math.Discretization to
  edu.wpi.first.math.system.Discretization
* Sort Java Discretization to match C++ function order
* Add tests for Java Discretization
  * Required adding Runge-Kutta time-varying impl to tests
* Move C++ Runge-Kutta time-varying impl to tests only
  * Users don't need it

wpimath/src/test/native/include/frc/system/RungeKuttaTimeVarying.h

@@ -0,0 +1,34 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include <array>
+
+#include "Eigen/Core"
+#include "units/time.h"
+
+namespace frc {
+
+/**
+ * Performs 4th order Runge-Kutta integration of dy/dt = f(t, y) for dt.
+ *
+ * @param f  The function to integrate. It must take two arguments t and y.
+ * @param t  The initial value of t.
+ * @param y  The initial value of y.
+ * @param dt The time over which to integrate.
+ */
+template <typename F, typename T>
+T RungeKuttaTimeVarying(F&& f, units::second_t t, T y, units::second_t dt) {
+  const auto h = dt.to<double>();
+
+  T k1 = f(t, y);
+  T k2 = f(t + dt * 0.5, y + h * k1 * 0.5);
+  T k3 = f(t + dt * 0.5, y + h * k2 * 0.5);
+  T k4 = f(t + dt, y + h * k3);
+
+  return y + h / 6.0 * (k1 + 2.0 * k2 + 2.0 * k3 + k4);
+}
+
+}  // namespace frc