Fix [issue #168] virus_on_network not updating

[DrEntropy]

As stated in #168 virus_on_network visualization does not update as the model advances. The issue is that SolaraViz watches model._step which doesn't advance in this model because the model doesn't use a scheduler. This is fixed by calling self._advance_time() in model:step(). This is a one line change.

[EwoutH]

Thanks for the PR. I’m holding of with this PR until we have a bit more clarity on projectmesa/mesa#2223 / projectmesa/mesa#2227.

If it isn’t resolved next week, we’re either need to revert #161 or do this change for all models.

I hope it doesn’t come to that. Feel free to join any of the discussions.

[EwoutH]

Thanks for the PR, we've reverted #161 (done in #170), so we're back to the schedulers right now, until we have our conceptual view of time fully sorted out.

Thanks anyways!