Deprecate sample_rate

[skjerns]

As discussed in #247 , I deprecated sample_rate and replaced all of its occurences with sample_frequency.

Frequency is usually measured in Hz, which is defined per second and is most consistent with other libraries (e.g. mne).

Summary

• There are errors raised when users try to use sample_rate - this is to ensure everyone finds out about the change without googeling
• I added a new routine to calculate record_duration - it should find an optimal value if one exists and let users not worry about the sample_rate as it is defined in edflib but to simply sey sample_frequency and be done
• It's still possible to overwrite the value of record_duration if ppl actually need to for obscure reasons

Additionally, needed to deprecate 3.7 for the test suite as it's not available for ubuntu24

fixes #198
closes #247

[skjerns]

@cbrnr @cfranklin11 @LucaCerina @raphaelvallat @DimitriPapadopoulos if any of you has time to have a look, that would be great :)

[raphaelvallat]

LGTM 👍

[LucaCerina]

Good also for me

[cbrnr]

Just a few minor suggestions, otherwise LGTM (although I didn't check the record duration calculation since I don't have the time at the moment).

[skjerns]

Found something strange: Even though the record_duration can be 8 ASCII chars according to the definition, it truncates after 7 in our library.

f.setDatarecordDuration(0.123456)
...
AssertionError: read 0.12345 != expected 0.123456

In the raw header there is an empty space

this issue might be related #242. I suggest fixing this in a future PR.

[skjerns]

Given that the the requested changes were minor, I'll merge this one :) thanks for reviewing!