Else clauses for matching structure elements when no converter matches on a specific level

Summary

The idea is to provide a treatment for all structure elements that are not matched by anything else.

The example is related to crawling BIDS data structures:

        session_level:
          type: Directory
          match: ^(ses-(?P<seslabel>[a-zA-Z0-9]+))$
          debug_match: True
          records:
            Session:
              label: "NA"
            Subject:
              Session: +$Session: 
        session_else:
          type: BIDSMissingSessionDirectory
          match: <<ELSE>>  # meaning: match if none of the converters on the same level matches
          debug_match: True
          records:
            Session:
              label: "NA"
            Subject:
              Session: +$Session
          subtree: