Variable extraction: allow multiple periods
function "extract.Var", argument "period": allow providing a dataframe P with X rows, 2 columns.
|"01-01-1965" "01-01-2000"|
P = |"01-01-1965" "01-01-2010"| |"01-01-1965" "01-01-2020"|
Dataframe P will be transformed into a group "period" in the data (3 levels in this exemple). And the variable extraction will be performed accordingly to that group.