More documentation for "sample frequency"

[DimitriPapadopoulos]

It turns out "sample frequency" is actually the number of samples per channel/signal in EDFlib:
https://www.teuniz.net/edflib/doc/edflib_8h.html#a11d03849cecc17a2599125a1a102b69e

We inherit this strange definition, and need to explain it.

Fixes #199.

[skjerns]

I see. This is all a big mess.

I would still favour to express sampling_frequency in Hz (samples/second) and not in samples/block_size and therefore deviate from the definition in edflib.
I don't think anybody thinks in terms of samples/block_size, as the block_size is irrelevant outside of the technicial file definitions of the EDF file.

Somehow, pyedflib originally decided to use sample_rate instead of sample_frequency, but kept the strange definition of sample_frequency from edflib as samples/duration. My idea to introduce sample_frequency was to keep backwards compatibility with previous code that might use sample_rate and set a specific record_duration. This most likely just complicated things 😅