Added test to move a ship and fixed associated issues.

Fixed loading of sectors by name, since this has to include the cluster name in the search term.
Fixed Game.assign_child to remove current parent if it exists.
Fixed Game.get_available_warps to use get_parent instead of referencing parent object directly.
Fixed Game.move_ship to work properly with entities.
Correctly implemented Entity.remove_child, since the list.remove method does NOT return anything (as I thought it returned a boolean).