make Calc function WEEKNUM compliant with ODFF1.2,
provide backward compatibility for Calc function WEEKNUM,
add unit tests for ISOWEEKNUM, WEEKNUM and backward compatibility.

Change-Id: I63af5543cea2f470d710462e55404ac754022c89
Reviewed-on: https://gerrit.libreoffice.org/18760Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>