Fixes for Seisan + Nordic files: parsing of int16 waveform files, edge cases in event files, compact event files

[flixha]

What does this PR do?

• obspy.io.nordic:
  • Parse ID-lines (type I) into event.extra['nordic_event_id]
  • Support reading of compact Nordic files (only containing header lines)
  • Write amplitude with highest possible precision
  • Fix writing / reading of high-frequency IAMLHF amplitudes
  • Fix parsing of BAZ when they precede the associated phase pick in Nordic file
  • Fix bug where check for nordic format version crashed on long phase names
• obspy.io.seisan:
  • Fix parsing of files written in int16 (previously read as int32 which resulted in waveforms that looked very similar to the actual waveforms, but had wrong number of samples and wrong amplitudes)

Why was it initiated? Any relevant Issues?

This PR takes care of some additional, rather rare edge cases for reading of Seisan waveform files and Nordic event parameter files. There were not related open issues that this PR resolves.

I realize this PR now combines a few more commits than I originally anticipated, so sorry for that. Let me know if you'd rather have it split up.

PR Checklist

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[megies]

Right now this might be the only instance setting anything on extra but that might change.. might be safer to do something like

extra = event.setdefault('extra', {})
extra['nordic_event_id'] = ...

[megies]

This might be OK but indexing is used 4 times on that line, so it might or nmight not be too "catch all" of an exception handling? Up to you though, I don't know the details here too much.

[megies]

I don't know this format at all, so I only have a few comments after briefly looking at this. Otherwise it's looking good to me (again without knowing or using that format).

Two things to consider though:

• I feel like this is a bit of a mix of things that could go into maintenance branch and some other things that feel like changing too much for that and should rather go into master. It's fine for me to have all this together in this PR but maybe we should rather merge into master then? (can be easily changed up top, only changelog would need moving after a quick rebase on master)
• Current status is we might still support Python 3.8 for the next set of bugfix+feature release (see discussion in Release 1.5.0 #3228 and raise min versions of numpy, scipy, mpl in setup.py #3087) and then drop it right after, so maybe it might be good to work around that new removeprefix() convenience method

[flixha]

Thanks a lot for the review!
I will get to fix your suggestions, may just need a few days until I get on it.
I understand and agree that this may be better suited for merging into master for the next release - after all it fixes relatively rare edge cases while the new functionality would have better gone into a new release anyways.