Refactor weighted aggregation for hhs as a method in the geomapper

[minhkhul]

This will definitely work, though it would be better if this were implemented as a method in the geomapper itself, for 2 reasons: reuse, and speed. aggregate_by_weighted_sum has to create the weights at every single time_value, which means it's definitely slower.

Originally posted by @dsweber2 in #2036 (review)