It is needed to use the right string representation of field value.
During export this representation is available directly, next to
the actual value. During import we can use the number format attached
to the corresponding pivot field.

Change-Id: Ie8f731f6c72d7f55f43c318e52d63dc4e993d9c0
Reviewed-on: https://gerrit.libreoffice.org/42069Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>