Convert Aggregator tasks to one-aggregator-per-task

[thcrock]

Our aggregation tasks are too big. They are currently scoped to everything needed to produce a particular dataset, which sometimes ends up being multiple classifiers, a skill extractor, etc. This can end up using a lot of memory. And if we want to output multiple datasets that use the same aggregation task, all that work ends up being done again by each dataset generator.

We should really split each of these aggregation tasks into its own Airflow task, which can then be merged together afterwards.

We can do most of this here in the Airflow repository, by splitting tasks like GeoTitleCount into smallers ones that output smaller CSVs, and adding a new merge task that combines the CSVs into the output currently produced.