[wpimath] Move math functionality into new wpimath library (#2629)

The wpimath library is a new library designed to separate the reusable math functionality
from the common utility library (wpiutil) and the hardware-dependent library (wpilibc/j).

Package names / include file names were NOT changed to minimize breakage.  In a future year
it would be good to revamp these for a more uniform user experience and to reduce the risk
of accidental naming conflicts.

While theoretically all of this functionality could be placed into wpiutil, several pieces
of this library (e.g. DARE) are very time-consuming to compile, so it's nice to avoid this
expense for users who only want cscore or ntcore.  It also allows for easy future separation
of build tasks vs number of workers on memory-constrained machines.

This moves the following functionality from wpiutil into wpimath:
- Eigen
- ejml
- Drake
- DARE
- wpiutil.math package (Matrix etc)
- units

And the following functionality from wpilibc/j into wpimath:
- Geometry
- Kinematics
- Spline
- Trajectory
- LinearFilter
- MedianFilter
- Feed-forward controllers

wpimath/src/main/native/include/frc/kinematics/DifferentialDriveOdometry.h

@@ -0,0 +1,87 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2019-2020 FIRST. All Rights Reserved.                        */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include "DifferentialDriveKinematics.h"
+#include "frc/geometry/Pose2d.h"
+#include "units/length.h"
+
+namespace frc {
+/**
+ * Class for differential drive odometry. Odometry allows you to track the
+ * robot's position on the field over the course of a match using readings from
+ * 2 encoders and a gyroscope.
+ *
+ * Teams can use odometry during the autonomous period for complex tasks like
+ * path following. Furthermore, odometry can be used for latency compensation
+ * when using computer-vision systems.
+ *
+ * It is important that you reset your encoders to zero before using this class.
+ * Any subsequent pose resets also require the encoders to be reset to zero.
+ */
+class DifferentialDriveOdometry {
+ public:
+  /**
+   * Constructs a DifferentialDriveOdometry object.
+   *
+   * @param gyroAngle The angle reported by the gyroscope.
+   * @param initialPose The starting position of the robot on the field.
+   */
+  explicit DifferentialDriveOdometry(const Rotation2d& gyroAngle,
+                                     const Pose2d& initialPose = Pose2d());
+
+  /**
+   * Resets the robot's position on the field.
+   *
+   * You NEED to reset your encoders (to zero) when calling this method.
+   *
+   * The gyroscope angle does not need to be reset here on the user's robot
+   * code. The library automatically takes care of offsetting the gyro angle.
+   *
+   * @param pose The position on the field that your robot is at.
+   * @param gyroAngle The angle reported by the gyroscope.
+   */
+  void ResetPosition(const Pose2d& pose, const Rotation2d& gyroAngle) {
+    m_pose = pose;
+    m_previousAngle = pose.Rotation();
+    m_gyroOffset = m_pose.Rotation() - gyroAngle;
+
+    m_prevLeftDistance = 0_m;
+    m_prevRightDistance = 0_m;
+  }
+
+  /**
+   * Returns the position of the robot on the field.
+   * @return The pose of the robot.
+   */
+  const Pose2d& GetPose() const { return m_pose; }
+
+  /**
+   * Updates the robot position on the field using distance measurements from
+   * encoders. This method is more numerically accurate than using velocities to
+   * integrate the pose and is also advantageous for teams that are using lower
+   * CPR encoders.
+   *
+   * @param gyroAngle The angle reported by the gyroscope.
+   * @param leftDistance The distance traveled by the left encoder.
+   * @param rightDistance The distance traveled by the right encoder.
+   * @return The new pose of the robot.
+   */
+  const Pose2d& Update(const Rotation2d& gyroAngle, units::meter_t leftDistance,
+                       units::meter_t rightDistance);
+
+ private:
+  Pose2d m_pose;
+
+  Rotation2d m_gyroOffset;
+  Rotation2d m_previousAngle;
+
+  units::meter_t m_prevLeftDistance = 0_m;
+  units::meter_t m_prevRightDistance = 0_m;
+};
+}  // namespace frc