Add DemandCentreNode

[s-simoncelli]

All the demand nodes I use constraint the max_flow using an aggregated parameter that multiplies the node base demand, a demand profile and a scaling factor to scale the model Deployable Output. I was wondering whether anyone else is interested in adding a DemandCentreNode that internally uses the AggregatedParameter.

[jetuk]

I think my only concern is whether this is too niche. I know it is common, but I can also imagine, for example, other uses with a demand that was a PCC multiplied by a population. Perhaps if we had a less catch-all name it could be added. I.e. it should leave room for other demand centre nodes to be added. Or the demand "model" be one of several alternatives?