Although year basis 2 is not supported by Excel, it is defined in ODFF1.2
for AMORLINC and is supported by Gnumeric.

Change-Id: Iacf60dbb7c0502338998f7ff22233c5f8b0b7146
Reviewed-on: https://gerrit.libreoffice.org/29188Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>