Make DigitalOutput a DigitalSource (#2054)

It's perfectly safe to use as a DigitalSource, and can enable some cool tricks.

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

@@ -18,7 +18,7 @@ import edu.wpi.first.wpilibj.smartdashboard.SendableRegistry;
  * Class to write digital outputs. This class will write digital outputs. Other devices that are
  * implemented elsewhere will automatically allocate digital inputs and outputs as required.
  */
-public class DigitalOutput implements Sendable, AutoCloseable {
+public class DigitalOutput extends DigitalSource implements Sendable, AutoCloseable {
   private static final int invalidPwmGenerator = 0;
   private int m_pwmGenerator = invalidPwmGenerator;
 
@@ -76,6 +76,7 @@ public class DigitalOutput implements Sendable, AutoCloseable {
    *
    * @return The GPIO channel number.
    */
+  @Override
   public int getChannel() {
     return m_channel;
   }
@@ -177,4 +178,34 @@ public class DigitalOutput implements Sendable, AutoCloseable {
     builder.setSmartDashboardType("Digital Output");
     builder.addBooleanProperty("Value", this::get, this::set);
   }
+
+  /**
+   * Is this an analog trigger.
+   *
+   * @return true if this is an analog trigger
+   */
+  @Override
+  public boolean isAnalogTrigger() {
+    return false;
+  }
+
+  /**
+   * Get the analog trigger type.
+   *
+   * @return false
+   */
+  @Override
+  public int getAnalogTriggerTypeForRouting() {
+    return 0;
+  }
+
+  /**
+   * Get the HAL Port Handle.
+   *
+   * @return The HAL Handle to the specified source.
+   */
+  @Override
+  public int getPortHandleForRouting() {
+    return m_handle;
+  }
 }