Model: Don't leave agents when there is no more data for them

Previously if we added three windfarms, we could never have
fewer than three going forward. Now we kill the additional
ones if we get fewer windfarms sent to us.
Model: save powerline group info, for future serialization
Model: Add very simple serialization demo

This simply JSON encodes the agents in the model
(excluding the powerlines, for now), and btoa-encodes
them.

If we load in a model with the encoded data in the hash,
we parse the data and add the agents, and then (for now)
block the OpenCV data as that will destroy the loaded agents.

In the future, we would want to save the data to a database,
instead of encoding it in the url, naturally.