fix for clmn not found bug

Nested outer joins could result in a column not found error thrown by the ExpressionAssembler in the compiler. The error occurred when a MapJoinProject was inserted, it had to shield the columns from subqueries. The proposed solution tracks whether the MapJoinProject visitor is visiting a node within a subquery, if so it does not add its column.

The commented out test case appears to be a different bug as described in: https://trello.com/c/CH388bkQ/837-lost-track-of-column-when-a-nested-subquery
This is going to happen on leaving the first node inside the subquery, when you want it to happen on leaving the subquery itself. Need to check the type of n here, too.