DataFrame poor design

[remicousin]

          ```

a pd.DataFrame with columns "label" (dtype=string), "key" (string or int depending on the table), and "the_geom" (shapely.Geometry)

FWIW the fact that `key` is sometimes a string and sometimes a number is a bad design that tends to lead to bugs. But that's not at issue right now.

_Originally posted by @aaron-kaplan in https://github.com/iridl/python-maprooms/pull/528#discussion_r2025580442_