fdo#73374 Trendline: correct R^2 for forced intercept

In case of forced intercept, R^2 must be calculated in a different way.
This patch calculates R^2 of trend line in the same way as LINEST
function.

Change-Id: I3ac361f014569261f05d513acb3428de5c7641ab
Reviewed-on: https://gerrit.libreoffice.org/7326
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>