Demonstrate metaballs

[blairlyons]

• [x] https://github.com/simularium/simulariumio/issues/96
• [x] https://github.com/simularium/simularium-viewer/issues/255

Use Case

As a user with agents that change shape over time, I want to visualize these agents without having to provide meshes for each agent at each time step.

Acceptance criteria

• Simularium files can contain data that the Viewer can use to calculate meshes in real time

Details

First investigate if this can be done performantly with metaballs, but consider other options as well.