/// <summary>
 /// Add days
 /// </summary>
 /// <param name="i"></param>
 /// <param name="days"></param>
 /// <returns></returns>
 /// <exception cref="ArgumentNullException"></exception>
 public static Instant AddDays(this Instant i, int days) => i.PlusDays(days);
 /// <summary>
 /// Add days
 /// </summary>
 /// <param name="i"></param>
 /// <param name="days"></param>
 /// <returns></returns>
 /// <exception cref="ArgumentNullException"></exception>
 public static Instant AddDays(this Instant i, double days) => i.PlusDays(days);