Dublicates of experimental data files

In GitLab by @hehnen1 on Nov 1, 2017, 16:01

During the propti_prepare process, files are copied to the new directory. If the experimental data file is the same for multiple simulation setups, used for one inverse modelling run, `shutil.py` raises the `SameFileError`.

We should think about if we want to force the user to have multiple single files for experimental data or if we want to account for it. Options to account for it, that I see right now, are:
* numbering the files
* understand that it is actually the same file and step over the copy process if it is already at the destination directory.