Data reading fixes

[JGuetschow]

Pull request

• A few changes in string processing.
• Some failing tests fixed

Do not review or merge yet, I still have to test this thoroughly on actual data to see how much it messes up my data processing.

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Description in a {pr}.thing.md file in the directory changelog added - see changelog/README.md for details

Description

Please provide a short description what your pull request does.

• A few changes in string processing. Most importantly the order of mapping to 0 and NaN has changed for string containing multiple string codes. The reason ist that this is mostly used for higher level sectors where the list of codes is just the list of all codes in the subsectors. In this case it makes more sense to have NaN for the higher level sector if at least one of the subsectors is NaN instead of zero if at least one of the subsectors is zero.
• Some tests for the gaps functions in the csg were failing because the default time format has changed (us instead of ns). They have been fixed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@467b9e9). Learn more about missing BASE report.
✅ All tests successful. No failed tests found.

Additional details and impacted files

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  Hits            ?     5227           
  Misses          ?      145           
  Partials        ?        0           

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[fuchsi-huber]

I forgot to check for open PRs before starting with the fixes and upgrades. The test fixes are now already part of main - do you want the remaining changes from this PR merged as well @JGuetschow ?