Remove cellular model stub

[p-snft]

We have an experimental stub for cellular energy system modelling that in my opinion should be replaced by proper filtering of nodes and the labeling system. I suggest removing that stub. Are there objections?

[p-snft]

@lensum, as you originally added this, I am looking forward for your feedback.

[lensum]

Seeing that this has probably caused more confusion than it brought benefits, I support this motion.

Unfortunately, I'm hard pressed for time right now, so I might not be able to help much with the removal. From the code-perspective, I think this is mostly the removal of two or three loops in the Model (or BaseModel) class.