Hardcode breaks for slave population

Thanks to a suggestion from Yoni Appelbaum. More legible breaks, but also
no need to compute the breaks. Change the implementation to return the class
rather than just an integer.
Loading text centered in SVG
Better breaks for population scale
Scale the county boundaries again
Fixed density scales
Calculate derivative properties at the beginning
Declaring variables
Use a map object for functions
Refactoring
Renamed properties