Provide a RefSkel.read() shortcut to read entire target skeleton

[phorward]

For several use-cases, this

studyworkflow_skel = conf.main_app.studyworkflow.viewSkel()

if not studyworkflow_skel.fromDB(skel["studyworkflow"]["dest"]["key"]):
    logging.error(f"""The studyworkflow {skel["studyworkflow"]["dest"]=} was meanwhile deleted?""")
    return

could be substituted by

studyworkflow_skel = skel["studyworkflow"]["dest"].read()

with nice exceptions being thrown when the skeleton is gone!

[phorward]

Fixed by #1193