Animate variables

[RuboGubo]

Add the ability to animate variables, similar to how desmos does it.

Why: It would help the user gain a better understanding of how a change in a variable affects a graph.

Follow on features: add the ability to export a video/gif of the animation.

[alexhuntley]

I like the idea. Exporting a video could be tricky however.

[astro-ray]

It would be nice to have a slider for parameters as well