Cannot read headers without reading full file

[klaff]

Currently, EDF.jl reads the header info and then checks against the file length and corrects the record_count if the file length doesn't match. For performance in certain situations, it would be beneficial to be able to read the header without reading through the rest of the file, or even when the full file length is not available.

[palday]

I think we could add a kwarg validate_record_count=true -- really highlights why this exists. There are a number of good and bad reasons why the record count in the header can be incorrect for real data.