Allow pipelines to be part of nodes in DataFile API

Fixes the problem Björn encountered in !974 (merged)

In the long-term, we should use the more advanced version from https://git.xfel.eu/dataAnalysis/exdf-tools.

@ahmedk @bjoerns

mentioned in merge request !974 (merged)

LGTM

merged

mentioned in commit aa3b64fb

Thanks for review

mentioned in merge request !979 (merged)

changed milestone to %3.12.6