allow exclusions from group duplicate search

[westram]

When searching duplicate groups from 2 or more trees, the results could be used to detect differences in taxonomy between trees.

It should be possible to exclude groups from the listed results by comparing group-specific values, e.g.:

• exclude clusters where all members have
  • same size
  • similar AID
  • equal-named direct parent
  • same nesting level

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related: #652