When pasting CSV data into existing document, we cannot use the
ScImportExport optimization for insertion, because the cells can already
have content (a grouped formula in this case). So use appropriate
ScDocument functions, even if that means the import is slower.

This is a regression from commit 93959db4
"use DocumentImport for csv import, related fdo#69006".

Change-Id: I92f70abca0542d796e3aa674b28a31053fff00d0