Set of commits to be tagged v0.18

Changelog

Major refactoring of method BackendExplorer.prepare_labels:

• argument allow_file_extensions to filter out input files based on file extension,
• argument single_input=True to pick a primary input file first, and then select the related files only,
• allow_file_extensions and single_input apply only when no metadata file is available,
• the filename metadata is used to identify the primary input file, which now comes first in the list of dependencies.