create a IClientSimulator to test volume rendering

[toloudis]

Create an IClientSimulator like the ones used to test points, curves, molecules etc in the example viewer. The simulator should just display one agent. (It will be the volume)

More detail:
make the default test viewer show a new test simulator. The new test simulator should just show one single agent. For now this agent will just be a sphere but this will be used as main testbed for volume data as we move forward. Mostly this can be copy/pasted from one of the other simulators (like the one that shows one single PDB molecule for example)

I suggest creating a new simularium-viewer "feature branch" to collect up these smaller tasks while we develop the prototype