Use field numbers in CSV import instead of indexes

For the common user, a field number is more intuitive than field index.
So let's use that in numbering CSV import fields.

Fixes #388

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Update manual to reflect CSV import numbering

Patch f4db392a changed CSV import to use field numbers instead of
indexes. Thus removing the now incorrect description of numbering from
user manual. Note, that screenshot should be updated as well.

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