animation between model updates

[jckr]

right now, there is no distinction between:

• the data of the model, which changes each time we run updateData,
• the visual attributes for rendering, which are passed to frames.

but it could be valuable to separate them and to be able to update the latter without necessarily changing the former.
for instance, we can imagine a simulation where we update data every 100ms, but want a smooth animation, so we want to re-render every 1/60s.