HarvesterNPC should spawn a biomass in-place after they die (#33)

Server/src/main/java/net/simon987/server/event/ObjectDeathEvent.java

@@ -0,0 +1,18 @@
+package net.simon987.server.event;
+
+/**
+ * Event dispatched by a GameObject who has needed callbacks on death
+ */
+public class ObjectDeathEvent extends GameEvent {
+    /**
+     * The GameObject type ID of object that init this event
+     */
+    private int sourceObjectId;
+
+    public ObjectDeathEvent(Object source, int sourceObjectId) {
+        setSource(source);
+        this.sourceObjectId = sourceObjectId;
+    }
+
+    public int getSourceObjectId() { return sourceObjectId; }
+}

Server/src/main/java/net/simon987/server/game/GameObject.java

@@ -251,4 +251,9 @@ public abstract class GameObject implements JSONSerialisable {
     public void setDead(boolean dead) {
         this.dead = dead;
     }
+
+    /**
+     * Called before this GameObject is removed from the world - defaults to doing nothing
+     */
+    public void onDeadCallback() { }
 }

Server/src/main/java/net/simon987/server/game/World.java

@@ -103,6 +103,7 @@ public class World implements JSONSerialisable {
             //Clean up dead objects
             if (object.isDead()) {
                 gameObjects.remove(object);
+                object.onDeadCallback();
                 //LogManager.LOGGER.fine("Removed object " + object + " id: " + object.getObjectId());
             } else if (object instanceof Updatable) {
                 ((Updatable) object).update();