ScanMeta: allow multiple metadata generators (i.e. with different registration...

ScanMeta: allow multiple metadata generators (i.e. with different registration names) to modify the same sub-tree of a metadata category (i.e. merge dictionaries instead of overwrite)
8 jobs for !3724 with 2769-scan-and-dataset-metadata in 107 minutes and 56 seconds (queued for 2 seconds)
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