Selection of required columns in query result

Currently the query result includes all existing header columns (and all their combined values). This is a bit spammy.

• Add another parameter to the query, listing the columns to include in the answer. It would be the same type as the aggregate parameter. The proposed name is columns.
• Don't include every column, but only the ones mentioned in aggregate or columns. Note that the user might include the same column in both and we want to list it only once in such case.

Note that this is to be placed on top of the code to be merged in !6 (merged).