FEATURE: life table aggregation function for non-age variables

[krpaulson]

Is your feature request related to a problem? Please describe.
We have agg_lt for aggregating life tables over age. However, a different approach is required when aggregating over other variables, like location hierarchy.

Describe the solution you'd like

1. mx * population --> deaths
2. ax * deaths --> death-weighted ax
3. aggregate deaths and death-weighted ax with sum function within hierarchyUtils::agg
4. death-weigthed ax / aggregated deaths = aggregated ax
5. aggregated deaths / aggregated population = aggregated mx
6. recompute any other life table parameters needed

Describe alternatives you've considered
Could also just write up a guide for this instead of writing a new hierarchyUtils::agg wrapper function.

Additional context
For IHME team members, see current FLT code here:
https://github.com/ihmeuw/ihme-modeling/blob/master/gbd_2019/mortality_code/full_life_tables/03_aggregate_lts.R