Game::Game: getPlayers() and getObjects() give non-const pointers

[Metabolix]

The functions Game::Game::getPlayers() and Game::Game::getObjects() should return boost::shared_ptr<const X> so that the caller can't modify the players and objects.

However, it isn't very efficient to duplicate all object pointers. This will probably be fixed when we implement some better way to traverse the objects (e.g. by location, such as getObjectsNearSomething()).