Reintegrated hyracks_fix_agg. Removed invorrect rule that adds a grouopby around a standalone aggregate. Replaced it with a rule for introducing a combiner to standalone aggregates.

git-svn-id: https://hyracks.googlecode.com/svn/branches/hyracks_asterix_stabilization@1829 123451ca-8445-de46-9d55-352943316053