improve "Merge classification datasets" algorithm

[Andreas Janz]

Requested by and discussed with @Florian Katerndahl .

• allow a list of datasets to be merged, not only 2
• support the usecase, where subdatasets are sampled using a tiling-scheme (e.g. FORCE data), and individual tiles may not have all categories. Merging datasets with non-matching category lists should also work

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[Andreas Janz]

resolves #1028

→ <<cset 07138ee1196d>>