Feature request: Add universe to output of load_variables()

[szimmer]

If it's available in the API, add the universe of each table to the output.

For example, tables can be limited to some of the following:

• Workers 16 years and over
• Workers 16 years and over who did not work from home
• Population 25 years and over
• Population 15 years and over
• Population in households

and more. It's always good to know what the universe (denominator) of each table is.