Add a flag to exchange DataFrame column names by labels (currently are question IDs)

Extracted task from issue #51:

• "better column names (from short texts?!)"

Here we need to exchange column names which are question IDs by default by column names taken from labels if a respective flag is given.

Another task could be to allow multi-language labels. If no language is specified one could use the general label. (related to #207)

We need to make sure that this won't break anything else, e.g. the plotting, if the columns are not question IDs anymore.