jupyterviz: best practices for model layout

[rlskoeser]

current best practices documentation for mesa recommends model layout with model.py, server.py and run.py . Is there a recommendation for models using jupyterviz? Would you recommend only creating model.py and app.py ?

[rht]

Prior discussion: #1269. I suppose that there are 2 conventions until the experimental JupyterViz has become sufficiently stable.