[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/spline/CubicHermiteSpline.h

@@ -0,0 +1,85 @@
+/*----------------------------------------------------------------------------*/
+/* 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 <array>
+
+#include <Eigen/Core>
+
+#include "frc/spline/Spline.h"
+
+namespace frc {
+/**
+ * Represents a hermite spline of degree 3.
+ */
+class CubicHermiteSpline : public Spline<3> {
+ public:
+  /**
+   * Constructs a cubic hermite spline with the specified control vectors. Each
+   * control vector contains info about the location of the point and its first
+   * derivative.
+   *
+   * @param xInitialControlVector The control vector for the initial point in
+   * the x dimension.
+   * @param xFinalControlVector The control vector for the final point in
+   * the x dimension.
+   * @param yInitialControlVector The control vector for the initial point in
+   * the y dimension.
+   * @param yFinalControlVector The control vector for the final point in
+   * the y dimension.
+   */
+  CubicHermiteSpline(std::array<double, 2> xInitialControlVector,
+                     std::array<double, 2> xFinalControlVector,
+                     std::array<double, 2> yInitialControlVector,
+                     std::array<double, 2> yFinalControlVector);
+
+ protected:
+  /**
+   * Returns the coefficients matrix.
+   * @return The coefficients matrix.
+   */
+  Eigen::Matrix<double, 6, 3 + 1> Coefficients() const override {
+    return m_coefficients;
+  }
+
+ private:
+  Eigen::Matrix<double, 6, 4> m_coefficients =
+      Eigen::Matrix<double, 6, 4>::Zero();
+
+  /**
+   * Returns the hermite basis matrix for cubic hermite spline interpolation.
+   * @return The hermite basis matrix for cubic hermite spline interpolation.
+   */
+  static Eigen::Matrix<double, 4, 4> MakeHermiteBasis() {
+    // clang-format off
+    static auto basis = (Eigen::Matrix<double, 4, 4>() <<
+     +2.0, +1.0, -2.0, +1.0,
+     -3.0, -2.0, +3.0, -1.0,
+     +0.0, +1.0, +0.0, +0.0,
+     +1.0, +0.0, +0.0, +0.0).finished();
+    // clang-format on
+    return basis;
+  }
+
+  /**
+   * Returns the control vector for each dimension as a matrix from the
+   * user-provided arrays in the constructor.
+   *
+   * @param initialVector The control vector for the initial point.
+   * @param finalVector The control vector for the final point.
+   *
+   * @return The control vector matrix for a dimension.
+   */
+  static Eigen::Vector4d ControlVectorFromArrays(
+      std::array<double, 2> initialVector, std::array<double, 2> finalVector) {
+    return (Eigen::Vector4d() << initialVector[0], initialVector[1],
+            finalVector[0], finalVector[1])
+        .finished();
+  }
+};
+}  // namespace frc