Point-in-time (vintage) economic data from FRED's ALFRED archive — what an economic figure was AS FIRST REPORTED / as known on a past date, before later revisions. Give a seriesId (e.g. GDP, GDPC1, PAYEMS, INDPRO) and an asOf date to get the observations exactly as they stood on that date; omit asOf to get the current (latest-revised) values. Also returns the series' vintage (revision) dates — eve