Change location_id to pop_id

[AHsu98]

location_id in dataframe splitting is a bad term--we should change it to the term pop_id or population_id to generally refer to the subpopulation that we are splitting (whether it is a country, or age group, both, etc.) Also, we can have that as an input (using possibly a list with indexing for multiple columns at once, since we have to send it to a tuple first if we want to select by both age and location).

[AHsu98]

Actually, demographic_id is probably the best terminology as the most general term for a group of people sharing some similar traits.