What this MR does / why does DuMux need it:
The extractlinedata.py had a bug when using it with multiple vtk files as input arguments together with a name for the output .csv file. It was writing all data into a single .csv file, that contained only the data of the last given vtk file. If the name is omitted, it uses the file names (without extension) of the given .vtk files. This MR simply adds an index to the resulting .csv files.