Fixing pdf sampling files path

This MR changes the path of the PDF_sampling folder, where all the intermediate files of the pdf sampling are saved, from the sample_path to the file_path (In the current example this would be from hybrids to preprocessed. With this, you can run the --prepare step once for all classes you might wanna use (the resulting files are saved in hybrids) and then run different preprocessings (different classes for example) with those files in hybrids. If you would try to run the pdf sampling how it is now, the PDF_sampling folder (which changes) would be overwritten every time.