Obtain the stages of a given variable in a staged event tree object.
stages(object, var = NULL)an object of class sevt.
name of one variable in object.
If var is specified, it returns a character vector with
stage names for the given variable
(that is object$stages[[var]].
Otherwise, If var is not specified, stages
returns a list of character vectors containing the stages associated
to each variable in the model (that is object$stages).