Create function for aggregating person to other levels

[MaxGhenis]

Should take as args:

• ID
• Other groupbys, which should be unique by ID
• Aggregates to be summed
• Maybe some columns that are only for the first person?

Then does a groupby and checks that it's one per ID

[nikhilwoodruff]

So far we've got tax unit summation in tools/general, though this should be trivial to generalise.