Add SlewRateLimiter class (#2192)

This is extremely useful for implementing various "ramping" functions
(such as voltage ramps, setpoint ramps, etc). Usage is straightforward;
it behaves like all of our other filter classes. C++ version is unit-safe.

wpilibj/src/test/java/edu/wpi/first/wpilibj/SlewRateLimiterTest.java

@@ -0,0 +1,29 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2019 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;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class SlewRateLimiterTest {
+  @Test
+  void slewRateLimitTest() {
+    SlewRateLimiter limiter = new SlewRateLimiter(1);
+    Timer.delay(1);
+    assertTrue(limiter.calculate(2) < 2);
+  }
+
+  @Test
+  void slewRateNoLimitTest() {
+    SlewRateLimiter limiter = new SlewRateLimiter(1);
+    Timer.delay(1);
+    assertEquals(limiter.calculate(0.5), 0.5);
+  }
+}