Better handling of types in CSV outuput.

[petamaj]

When a vector is passed to the csv output, multiple lines are generated with each of the values being on own line, other columns intact. There is no warning this is happening and the behavior is a bit surprising.

How about requiring that when multiple values per item are passed to the CSV writer, the user must also indicate how these should be reported (i.e. multiple rows as above, multiple columns, joined by sth, or whatnot, just to make this explicit. Or if this can't be done, just make the multiple rows per item an explicit choice?