Add Transform2d::Inverse() (#2407)

This is useful for undoing transformations. One application my FRC team
found was converting perspective n-point data from a "camera to target"
coordinate frame transformation to a "target to camera" coordinate frame
transformation.

wpilibj/src/test/java/edu/wpi/first/wpilibj/geometry/Transform2dTest.java

@@ -0,0 +1,36 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+package edu.wpi.first.wpilibj.geometry;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertAll;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class Transform2dTest {
+  private static final double kEpsilon = 1E-9;
+
+  @Test
+  void testInverse() {
+    var initial = new Pose2d(new Translation2d(1.0, 2.0), Rotation2d.fromDegrees(45.0));
+    var transformation = new Transform2d(new Translation2d(5.0, 0.0),
+        Rotation2d.fromDegrees(5.0));
+
+    var transformed = initial.plus(transformation);
+    var untransformed = transformed.plus(transformation.inverse());
+
+    assertAll(
+        () -> assertEquals(initial.getTranslation().getX(), untransformed.getTranslation().getX(),
+                           kEpsilon),
+        () -> assertEquals(initial.getTranslation().getY(), untransformed.getTranslation().getY(),
+                           kEpsilon),
+        () -> assertEquals(initial.getRotation().getDegrees(),
+                           untransformed.getRotation().getDegrees(), kEpsilon)
+    );
+  }
+}