A node is a set of different components that are required by a system.
A system can request a collection of nodes from the engine.
Subsequently the Engine object creates a node for every entity that has _all_ of the components
in the node class and adds these nodes to the list obtained by the system.
The engine keeps the list up to date as entities are added
to and removed from the engine and as the components on entities change.</p>
The base class for a node.
A node is a set of different components that are required by a system.
A system can request a collection of nodes from the engine. Subsequently the Engine object creates a node for every entity that has _all_ of the components in the node class and adds these nodes to the list obtained by the system.
The engine keeps the list up to date as entities are added to and removed from the engine and as the components on entities change.</p>