Add group nodes (composition) example

As per last call, the composition will be visualised as a square that contains all the nodes inside.
We need to prototype this component, keeping in mind that:

when we drag and drop the compositions all the content will move with it.
I can still drag and drop any single node inside
if a node gets dropped outside the composition then it's not part of that anymore.
The focus of this card is more on the functional side rather than the concept of composition itself within psyneulink, so the 3 key points above are the things that we should focus implementation wise.