The Game is now forbidden from begin copied explicitly.

It wasn't able to be copied before because of the unique_ptr member, but
this will keep it that way regardless of change. Soon expensive-to-copy
things will be stored in the game and we can't have the settings, etc
becoming fragmented throughout the game states, etc.
Added a Json::Value to the Game structure.
Added a function that finds the containing node of an object based on its volume.
Added a function to get a volume from a json object.
A section of the remove function is now cleaner.
The remove function now asserts the fact that children *are* being removed.
The insert function now prevents ids to be added twice to a node.
Simplified the find_containing_nodes function and the test.