Merge branch 'fix_single_frame' into '2.1'

Fix single frame

See merge request !243 (merged)

(cherry picked from commit bec26331)

ad38fbaa volume: single frame volume: clean output files that could be used. 8a23bf14 Volume: move _clean_output_data to public API 95cc01a3 VolumeSingleFrameBase: rename 'ignore_existing_files' to... 291b7f69 remove_existing_data_files: allow url to be None 7f826207 remove_existing_data_files: handle use case 'url' is None

Co-authored-by: payno henri.payno@esrf.fr

assigned to @payno

enabled an automatic merge when all merge checks for fe789e3d pass

merged

mentioned in commit e9dc8584