Returns the next achievement for an achievement which is part of a series
See also Achievement functions.
Signature:
nextID, completed = GetNextAchievement(id)
Arguments:
id- The numeric ID of an achievement (number)
Returns:
nextID- If the given achievement is part of a series and not the last in its series, the ID of the next achievement in the series; otherwise nil (number)completed- True if the next achievement has been completed; otherwise nil (boolean)