These functions have been introduced with Excel 2010, but were not
supported yet in calc.
COVARIANCE.P (population) replaces the COVAR function, but the COVAR
function remains present (in Excel as well as in calc).
COVARIANCE.S (sample) is a new function.

Change-Id: If5501b4090fb716adfb3d121c7898528fd1b7ad4
Reviewed-on: https://gerrit.libreoffice.org/6135Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>