keep only filename as id in DocCollection

[erwanm]

issue migrated from original private gitlab repo

• In the standard case of loading from a directory with createDatasetsFromParams, the path is useless.
• Additionally, when storing the id in pre-sim values files (in Impostors verif method), the full path is stored, which would cause problems if the file is later read from a different directory.
• However there might be cases where it's necessary to keep the full path, I'm not sure. In this case maybe the path should be removed locally when saving pre-sim values, and re-added locally when loading the same file (i.e. it still works if the path changes between saving and loading)