cubicweb-brainomics #3446639 ExternalResources -> FileSet, FileEntry [open]

Change ExternalResources to FileSet across all Brainomics cubes, typically to be able to handle series of images, each image made of two files.

prioritynormal
typeenhancement
done in<not specified>
closed by<not specified>
patch[schema, views] ExternalResources -> FileSet + FileEntry, closes #3316345 [applied]