docs: def days(date1, date2):
      """
      returns a lambda that determines the number of days between the two dates
      the dates can be strings (column names) or actual dates
      e.g. Aggregate(days('date', today), ['min','max'], 'days_since')
      TODO: should there be a Days(AggregateBase) for this? handle naming
      """
      return lambda df, date1=date1, date2=date2: _days(df, date1, date2)