This function obtains the position structure associated with a stratified CEG.
StratifiedCegPosition(stage, num.category, num.situation)(list) - stage structure associated with a particular variable.
(vector) - number of edges that unfold from stages associated with a particular variable.
(vector) - number of situations associated with each variable.
list of lists
First list level identifies a variable 'v'.
Second list level identifies a stage 'a' associated with a variable 'v'.
The third list level identifies the positions associated with a stage 'a' .
@seealso PositionLevel, PositionVector,
PositionStage, PairwisePosition