PIDController now supports composition (#976)

For example, an outer position feedback controller can now drive the
reference of an inner velocity feedback controller.

wpilibj/src/main/java/edu/wpi/first/wpilibj/PIDBase.java

@@ -25,7 +25,7 @@ import static java.util.Objects.requireNonNull;
  * and derivative calculations. Therefore, the sample rate affects the controller's behavior for a
  * given set of PID constants.
  */
-public class PIDBase extends SendableBase implements PIDInterface {
+public class PIDBase extends SendableBase implements PIDInterface, PIDOutput {
   public static final double kDefaultPeriod = 0.05;
   private static int instances = 0;
 
@@ -765,6 +765,20 @@ public class PIDBase extends SendableBase implements PIDInterface {
     }
   }
 
+  /**
+   * Passes the output directly to setSetpoint().
+   *
+   * <p>PIDControllers can be nested by passing a PIDController as another PIDController's output.
+   * In that case, the output of the parent controller becomes the input (i.e., the reference) of
+   * the child.
+   *
+   * <p>It is the caller's responsibility to put the data into a valid form for setSetpoint().
+   */
+  @Override
+  public void pidWrite(double output) {
+    setSetpoint(output);
+  }
+
   @Override
   public void initSendable(SendableBuilder builder) {
     builder.setSmartDashboardType("PIDController");