Population exposure support

Addresses part of https://bugs.launchpad.net/oq-platform/+bug/1187008. We still need to implement the third export option: only dwelling fractions.

This branch includes a lot of refactoring. Most significantly, we re-wrote the exposure data fetching queries. This was originally implemented as 3 or 4 queries, but we have reduced it to just 1 for national (admin 0) and 1 for subnational (admin 1-3).