Add warning for metadata truncation when writing to mseed

[jawooll]

What does this PR do?

Changes default behaviour when writing to mseed files to now print a warning if the provided metadata would be truncated during writing.

Why was it initiated? Any relevant Issues?

See issues/questions (fixes #2971, #2588).

PR Checklist

• Correct base branch selected? master for new features, maintenance_... for bug fixes
• This PR is not directly related to an existing issue (which has no PR yet).
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  Just add the "build_docs" tag to this PR.
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• All tests still pass.
• Any new features or fixed regressions are covered via new tests.
• Any new or changed features are fully documented.
• Significant changes have been added to CHANGELOG.txt .
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[yetinam]

Hey Jack! If I remember correctly, this truncation issue affects a few formats that you listed here seisbench/seisbench#54 . Even though mseed will be the most prominent one, I was wondering whether it would be reasonable to add warnings for all formats in one go.

[jawooll]

Hey Jannes, Good point! This requires a more involved PR though. Let's first see if the obspy team are open to the idea of logging such warnings when writing streams, and I could then look into extending this if required.

[d-chambers]

Let's first see if the obspy team are open to the idea of logging such warnings

Totally open to it, please run with it.

[jawooll]

Ok great thanks, I'll extend the PR to log warning on all appropriate formats.

[megies]

This looks pretty good to me, just a few things/thoughts..

• please add a changelog one-liner
• please actually test that the appropriate warning is shown, currently it only checks if any warning appears which could be anything (we have a helper context manager making this easy)
• currently this is in a really low-level spot, which is OK, I guess, but if you plan on extending it to other file formats, I'd just move the check to inside Stream.write(). That is the recommended high-level routine and I'd argue people going the discouraged low-level way are kind of on their own with detail issues like this. Having it in a high level place like that would make it easier to extend for more formats in the future, by just adding to some dictionary storing limitations per format code