[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/cpp/system/RungeKuttaTimeVaryingTest.cpp

@@ -0,0 +1,37 @@
+// 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.
+
+#include <gtest/gtest.h>
+
+#include <cmath>
+
+#include "frc/system/RungeKuttaTimeVarying.h"
+
+namespace {
+Eigen::Matrix<double, 1, 1> RungeKuttaTimeVaryingSolution(double t) {
+  return (Eigen::Matrix<double, 1, 1>()
+          << 12.0 * std::exp(t) / (std::pow(std::exp(t) + 1.0, 2.0)))
+      .finished();
+}
+}  // namespace
+
+// Tests RK4 with a time varying solution. From
+// http://www2.hawaii.edu/~jmcfatri/math407/RungeKuttaTest.html:
+//   x' = x (2 / (e^t + 1) - 1)
+//
+// The true (analytical) solution is:
+//
+// x(t) = 12 * e^t / ((e^t + 1)^2)
+TEST(RungeKuttaTimeVaryingTest, RungeKuttaTimeVarying) {
+  Eigen::Matrix<double, 1, 1> y0 = RungeKuttaTimeVaryingSolution(5.0);
+
+  Eigen::Matrix<double, 1, 1> y1 = frc::RungeKuttaTimeVarying(
+      [](units::second_t t, Eigen::Matrix<double, 1, 1> x) {
+        return (Eigen::Matrix<double, 1, 1>()
+                << x(0) * (2.0 / (std::exp(t.to<double>()) + 1.0) - 1.0))
+            .finished();
+      },
+      5_s, y0, 1_s);
+  EXPECT_NEAR(y1(0), RungeKuttaTimeVaryingSolution(6.0)(0), 1e-3);
+}