Field variables for CSV import

Field variables for the CSV XSLT import have disappeared at some point
during developing GUI for the CSV import. So adding them to the XSLT for
the field selections to have effect.

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Get information only from first line at each time

As some CSV log files seem to contain multiple sample lines for a single
time, we'll just grab the first one of them to avoid duplicates.

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Open CSV files only via the GUI

Since the CSV import transformation is now parametrized and does not
have any defaults, we need to use the CSV import GUI. Thus give an error
message if one is opening CSV file directly.

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>