Change base geography to blockgroups

[chunhochow]

Right now, the base geographies are a mix of tracts and blockgroups. We should change it to just use blockgroups (and blockgroup based census stats) directly.

This way, the EPC neighborhood names can also be associated with each blockgroup and shown on the tooltip box on the top right when you hover.
(because EPC neighborhoods are drawn over blockgroups, not tracts, so this method won't work as is on the current base geograpahies)