fix sample encoding for export

[kleinschmidt]

fixes #75 and cleans up a lot of crufty old code that's gotten out of sync with both Onda and EDF.

It adds a new internal function reencode_samples that takes Onda.Samples and returns new samples that are encoded in an EDF-friendly way (either directly as Int16/the requested type, or as something that can be converted losslessly to that type). It tries very hard to not change the encoding if it's possible, based on the typemin/max of the input sample_type, adn failing that, the acutal min/max of the encoded values (for <:Integer sample types). If it's not possible to losslessly represent the encoded values as Int16, it picks a new quantization setting based on the min/max of the decoded values (fitting those to typemin/max of Int16 or whatever teh desired type is) and returning the re-encoded samples.

It also adds much more thorough tests of the export behavior, testing every type that Onda declares is allowed in Onda.LPCM_SAMPLE_TYPE_UNION.

While adding tests, I found that it's not possible for EDF.jl to decode things where the physical min/max are anywhere near the Float32 min/max finite values, see beacon-biosignals/EDF.jl#67 As a result, some of the re-encoding tests do not hit the true typemin/max of the Onda sample type, but I think that's okay. We're still in much better shape here than we were.

I'm putting this on hold for now but want to get a draft PR up

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[palday]

mildly cursed, but makes sense

[kleinschmidt]

yup, hence the XXX comment above, wanted to make sure this was known to be cursed

[palday]

this seems unlikely but I guess it doesn't hurt 🤷

[kleinschmidt]

ah, this is actually necessary when you have input samples with Int8/UInt8 encoding (which is the "mildly cursed" bit you called out above)

[palday]

I kinda think that this should be a method of Onda.encode_sample. To that end, you might want to add a comment to the docstring that this is an internal function and could disappear in the future (e.g. if moved into Onda itself).

[kleinschmidt]

I kinda think that this should be a method of Onda.encode_sample. To that end, you might want to add a comment to the docstring that this is an internal function and could disappear in the future (e.g. if moved into Onda itself).

Hm, I'm not sure about this...it's not part of the Onda documented API so I'm a bit reluctant, and it's also not really doing the same thing (IIRC, Onda.encode_sample is actually doing the encoding given specified parameters; we're choosing parameters here and encoding a whole Samples, not just a single sample...)