SigMFile data_file is None from archive

[Deschain]

SigMFFile member data_file is None when creaeting a SigMFFile from archive. This ends up raising an exception when calling get_capture_byte_boundarys due to calling path.getsize with None.

sigmf-python/sigmf/sigmffile.py

Line 390 in bd26060

end_byte = path.getsize(self.data_file) - self.get_global_field(self.TRAILING_BYTES_KEY, 0)

I don't know if there are other cases.

[lukashenneke]

data_file being None probably leads to sample_count=0 as well, which triggers an end of file error in read_samples if count is not -1.

For sigmf==1.1.5, reading archives works fine with data_file and sample_count set correctly.
For sigmf==1.2.10, reading archives results in data_file=None and sample_count=0.

[Teque5]

Okay so with the latest updates to v1.4.0, read_samples and slicing does work correctly for archives & pairs, but archives still do raise an error when calling get_capture_byte_boundaries. I'll make a PR to fix.