corner cases in _read_str, _read_int and _read_float

[emiddell]

This PR addresses corner cases found in existing snirf datasets.

• _read_str can handle strings that are stored as arrays of bytes
• _read_[str,float,int] can handle empty datasets (this addresses Crash when validating data entries with 0 length #49)

[sstucker]

Thanks so much for the contribution!

The type hinting with the | you used is only supported past 3.10 and I don't really want to use it. Change it to something compatible with at least 3.9, ideally 3.6. We're not totally settled on version support but I don't want to roll forward to >3.10 just for hinting.

I also need to figure out why all the tests are failing. I'm working on a few major updates and look forward to getting this sorted out shortly

Thanks again for finding this and fixing it.

[sstucker]

I am leaning towards supporting Python >3.9 for numpy 2.0.0