Removed old testing code from game.cpp.
Fixed the implementation of Game_object_manager::Get(), since it did not work properly.
This commit fixes and improves the movement of Skiers.

Skiers position updating used to not take the elapsed time of that frame, so movement was not consistent. Also, the precision of T in the sf::Vector< T > that Skier uses to store its velocity was increased from float to long double.
Removed leftove #includes for testing in game.cpp, and also added a default option in the Game::Update_game_state() switch statement.