Optional area overlays (e.g. population socio-demographic characteristics)

[carlhiggs]

Extending the idea in #36, in addition to optional points-of-interest overlay, there should be an option to overlay area statistics. For example, of population socio-demographic attributes, and potentially, links between these areas. If an overlay area were clicked on, it would display a population summary. It might be that the population statistics are stored not in the vector tile itself, but dynamically retrieved upon clicking a specific area and generating an interactive chart that visualises aspects of that cohort. Potentially, could also show within area flows between sub-districts (e.g. home and work).

An overlay of this type will provide context to visualisations of baseline/intervention network modelling results.