Fixes for latest ouracat

[aulemahal]

Pull Request Checklist:

• pre-commit hooks are installed/active in my local clone ($ pre-commit install)
• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• (If applicable) Documentation has been added / updated (for bug fixes / features)
• If a merge request has been made in parallel to this PR in xscen-notebooks, it is merged and the submodules have been updated.
• HISTORY.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Adds "2sem" to the known "frequency" strings. It translates to "2W".
• Skip unreadable files in parse_directory(according to their permissions).
• New "_" prefix in patterns of parse_directory, these fields may include underscores and are included in the output.

Does this PR introduce a breaking change?

No.

Other information:

[RondeauG]

Looks good!