Additional layers: population displacement, count of households, etc.

[doug-leasure]

The dev API now includes the functionality to specify the column from the SQLite database ("Nhat" table) that you would like to query ("Pop" is default). We can store estimated parameters other than total population in these additional columns (vector of posterior predictions or single numbers). We have immediate applications in SSD to add counts of people displaced from/to each pixel, and also for GHA to add counts of housing units.