Allows the user to extract values from a l1celist object by using
  subscripts.
# S3 method for l1celist
[(x, …, drop = TRUE)
# S3 method for l1celist
[[(x, …, drop = TRUE)an object inheriting from class "l1celist".
a specification of indices -- see Extract.
logical defaulting to TRUE.  If only one model is
    subscribed, then it is returned as an object of class "l1ce".
    If drop=F, then an object of class "l1celist" is
    always returned.
an object of class "l1celist" or class "l1ce"
  extracted from the original list.