remove images2gif and vendor module

[aerispaha]

I think there is no reason to house this logic in swmmio. This was used in the past to generate animated data visualizations - cool stuff, but I don't think it makes sense to have a duplicate of the logic that exists in the github.com/isaacgerg/images2gif repository.

Instead, think we could show an example of how to leverage that code in an extended environment (with images2gif installed), rather than having it baked into the swmmio source code.