Initialization callback creation

[remikey]

We are considering adding an initialization callback (at least) on the Node class. This callback would be called before the onStart callback. Its content would be in charge of initializing the node's state, possibly avoiding the necessity of "startAndPause" trick.

Note: This issue is created partly in order to keep track of discussions.

[remikey]

I have made an implementation proposition on feature/56-initialization-method. What's in it:

• Topology.initialize() added It is automatically called by Topology.start() if needed.
• InitializationListener created
  • InitializationListener.onInit() is meant to be called after the Topology's initialization
  • As for the other listeners, it has been added to the Topology
• Node.onInit() added It is meant to be called after the Topology's initialization.
• TopologyLifeCycle UT added to specify the basics of the topology's life cycle
• A MainInit example has been provided

This is still modifiable.

[remikey]

@acasteigts , following the discussion in my office:

• Topology.initialize() does not set isStarted back to false anymore.
• removing the new ArrayList(...) copies when notifying listeners in the hope of speeding things a bit actually raised ConcurrentModificationExceptions. I have added io.jbotsim.core.XXXListenerTests classes in test-classes (thus needing to be run manually) to tests the associated behaviors.
• I have put in a separate commit the fact that you have a strong doubt on the fact that we can switch notifying the listeners that a Node has been added and notifying the Node that the topology has been initialized/started.

[remikey]

• rebased over #85 and #86
• take latest modifications into account