Build hierarchical source model

[duhaime]

Right now if a user queries for a term that appears more than 3000 times in a single day of their requested source, the wsk module gives up, as its only strategy for reducing the number of responses to a given query is is subdivide the date range (e.g. reduce the number of days to query until we hit the quantum, a single day).

We could instead provide a workaround for queries that have > 3000 hits in a single day. Given a request for a source id, we can find all of the publications in that source. Then we can identify the other source ids that contain the same publications, and can query those instead. In this way we can piece together the publication titles that comprise a source id.