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/main/java/edu/wpi/first/wpilibj/SlewRateLimiter.java

@@ -0,0 +1,67 @@
+/*----------------------------------------------------------------------------*/
+/* 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 edu.wpi.first.wpiutil.math.MathUtil;
+
+/**
+ * A class that limits the rate of change of an input value.  Useful for implementing voltage,
+ * setpoint, and/or output ramps.  A slew-rate limit is most appropriate when the quantity being
+ * controlled is a velocity or a voltage; when controlling a position, consider using a
+ * {@link edu.wpi.first.wpilibj.trajectory.TrapezoidProfile} instead.
+ */
+public class SlewRateLimiter {
+  private final Timer m_timer = new Timer();
+  private final double m_rateLimit;
+  private double m_prevVal;
+
+  /**
+   * Creates a new SlewRateLimiter with the given rate limit and initial value.
+   *
+   * @param rateLimit The rate-of-change limit, in units per second.
+   * @param initialValue The initial value of the input.
+   */
+  public SlewRateLimiter(double rateLimit, double initialValue) {
+    m_prevVal = initialValue;
+    m_rateLimit = rateLimit;
+    m_timer.start();
+  }
+
+  /**
+   * Creates a new SlewRateLimiter with the given rate limit and an initial value of zero.
+   *
+   * @param rateLimit The rate-of-change limit, in units per second.
+   */
+  public SlewRateLimiter(double rateLimit) {
+    this(rateLimit, 0);
+  }
+
+  /**
+   * Filters the input to limit its slew rate.
+   *
+   * @param input The input value whose slew rate is to be limited.
+   * @return The filtered value, which will not change faster than the slew rate.
+   */
+  public double calculate(double input) {
+    m_prevVal += MathUtil.clamp(input - m_prevVal,
+                                -m_rateLimit * m_timer.get(),
+                                m_rateLimit * m_timer.get());
+    m_timer.reset();
+    return m_prevVal;
+  }
+
+  /**
+   * Resets the slew rate limiter to the specified value; ignores the rate limit when doing so.
+   *
+   * @param value The value to reset to.
+   */
+  public void reset(double value) {
+    m_timer.reset();
+    m_prevVal = value;
+  }
+}