sim.output.phys_fields.animate function to create movies and GIF

[paugier]

I think it's a very nice feature of core fluidsim solvers to have this function. Users can very easily create simple 2d animations representing the flow simulated. It's very useful to "see what happens" and of course for communication (BTW, we don't use it enough).

Animations in fluidsim are based on this method: https://fluidsim.readthedocs.io/en/latest/generated/fluidsim.base.output.movies.html#fluidsim.base.output.movies.MoviesBase.animate

It would be very nice to have something similar for Snek. I don't know if it could be based on the code of fluidsim.base.output.movies.MoviesBase. If it could, we would have to move this class in fluidsim_core (super easy).

I don't include this issue in our 0.8.0 milestone, but it would be very useful for #92 since it would be very easy to create nice animated GIF that could be included in the README and in the doc. Such GIF would also be useful for communication on social media (see for example https://twitter.com/me3250).

[paugier]

I don't know if it could be based on the code of fluidsim.base.output.movies.MoviesBase

Actually, I think it could be based on this class. And for the solvers/meshes for which the pymech reader works, it should not be difficult.

[paugier]

See also https://github.com/jbusecke/xmovie