Do not add index column by default when exporting to CSV
As pointed out by @merveenoyan, default behavior of `Dataset.to_csv` adds the index as an additional column without name.

This PR changes the default behavior, so that now the index column is not written.

To add the index column, now you need to pass `index=True` and also `index_label=<name of the index colum>` to name that column.

CC: @merveenoyan 