Resolve "implement collision detection"

frontend/core/src/com/game/tankwars/CollisionDetection.java

frontend/core/src/com/game/tankwars/model/Box2dWorld.java

frontend/core/src/com/game/tankwars/model/Bullet.java

frontend/core/src/com/game/tankwars/model/FixtureData.java

frontend/core/src/com/game/tankwars/model/Tank.java

frontend/core/src/com/game/tankwars/model/Terrain.java

frontend/core/src/com/game/tankwars/view/GameScreen.java