MILESTONE: Next improvements

[blairlyons]

More example models: Vivarium and ReaDDy

• [ ] https://github.com/simularium/simularium-planning/issues/102
• [ ] https://github.com/simularium/simularium-planning/issues/90

We will add more example models to the website.

UI Polish

• [ ] https://github.com/simularium/simularium-planning/issues/46
• [ ] https://github.com/simularium/simularium-planning/issues/62
• [ ] https://github.com/simularium/simularium-planning/issues/63

We will continue to improve the UI.