Repair _combine_blocks_to_df_withdims()

With the introduction to allow requesting sub-dimensions the display of unit is broken for distrib_attributes2d() and correlation(). The problem lies in the function _combine_blocks_to_df_withdims(). We have the following issues there:

• Labels and original column names need to be treated separately, since a label equals the original column name with its unit appended
• Columns that are in several blocks are not handled correctly anymore. Due to the possibility of requesting dobj names and dobj column names there could be duplicate columns despite the request being diffrent.