preserve R data.frame row names

When converting from an R data.frame to a vtkTable, store their
row names (if any) in a string column at the beginning of the table.

Change-Id: Ie9bc73a8c83ae7aec46ce05bc0de849a1c0f3da2
Merge topic 'dataframe_rownames' into master

c06dd72d preserve R data.frame row names