STDEVP and STDEVPA functions are based on an entire population

Change-Id: Ieb1aa2e956061ce72afc2d5d804b95dabd20c4c8
Reviewed-on: https://gerrit.libreoffice.org/7552
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>