Add getClock method to Node interface (osrf#43)

Useful for getting the current time from the Clock instance.

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

Author: jacobperron

rcljava/src/main/java/org/ros2/rcljava/node/Node.java

@@ -38,6 +38,7 @@
 import org.ros2.rcljava.service.RMWRequestId;
 import org.ros2.rcljava.service.Service;
 import org.ros2.rcljava.subscription.Subscription;
+import org.ros2.rcljava.time.Clock;
 import org.ros2.rcljava.timer.Timer;
 import org.ros2.rcljava.timer.WallTimer;
 
@@ -46,6 +47,11 @@
  * A Node must be created via @{link RCLJava#createNode(String)}
  */
 public interface Node extends Disposable {
+  /**
+   * @return The nodes @{link Clock}.
+   */
+  Clock getClock();
+
   /**
    * return All the @{link Client}s that have been created by this instance.
    */

rcljava/src/main/java/org/ros2/rcljava/node/NodeImpl.java

@@ -220,6 +220,13 @@ private static native <T extends MessageDefinition> long nativeCreatePublisherHa
   private static native <T extends MessageDefinition> long nativeCreateSubscriptionHandle(
       long handle, Class<T> messageType, String topic, long qosProfileHandle);
 
+  /**
+   * {@inheritDoc}
+   */
+  public final Clock getClock() {
+    return this.clock;
+  }
+
   /**
    * {@inheritDoc}
    */