Simplify and/or better explain Message construction

[LourensVeen]

The messages sent in the example are a bit complicated, because we have a send statement with a port, and then a Message which has some structure, which in turn wraps a Grid which is another unfamiliar object.

Maybe we should remove the names from the Grid and just put in the numpy array directly? Documenting the indices is good practice, but also makes things more complicated.

Alternatively, some extra explanation would be good, either by the teacher (in teaching instructions) or in the lesson itself.