Allow FERC 714 eia_code values to be categorized differently in different years

[zaneselvans]

Right now we assume that each eia_code in the respondent_id_ferc714 table is associated with either a balancing authority or a utility, across all the years for which there is data. However, we could also categorize them differently on a per-year basis, since over the last 14 years it's possible that some of the entities have gone from reporting as utilities to BAs or vice versa.

It's unclear whether this is necessary, or if it would improve our overall ability to accurately re-allocate demand geographically.

The planning_area_description_ferc714 table might be helpful in figuring this out, as it lists the names of all the utilities which are associated with each value of respondent_id_ferc714 -- it seems like the respondents that are themselves utilities ought to just be associated with... themselves?

We can also compare our categorizations against the ones which were done by hand in Auffhammer et al., at least in outline.