Added LeftOuterUnnestMap operator.

 - Added LeftOuterUnnestMap operator to represent the left-outer-join semantics properly.

Change-Id: I4525899cf8e5e43551aa2ac2a78806ef6cc85e58
Reviewed-on: https://asterix-gerrit.ics.uci.edu/638
Tested-by: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Reviewed-by: Yingyi Bu <buyingyi@gmail.com>