Excel and Gnumeric treat this argument as boolean.
Financially, payment at beginning or at end of period are the only
existing options, i.e. there no other feasible options.

Change-Id: I7ba9fcdac69b9b0756cdf48abde001a44c8b4c88
Reviewed-on: https://gerrit.libreoffice.org/27612Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>