cubicweb-medicalexp #3694093 Remove filepath for measures [validation pending]

The different measure (GenericMeasure, Scan, GenomicMeasure, ...) should not have a filepath with the new FileSet/FileEntry schema.

Each measure entity should carry specific information about the measure itself, as it may have many results files. The files are stored:

• as "results_files" links;
• as "additional_files" links;