Dataset file in same metadata directory instead of local directory

[lgoix]

I think there is an issue with the dataset description in SigMF Specification Version v1.2.2
https://sigmf.org/#ssubsec:dataset

The Dataset file must be in the local directory, and this string MUST NOT include any aspects of filepath other than the filename.

If I am running a script in /home/user/ and I open the signal.sigmf-meta file in the directory /media/usb1/, the specification is telling us the dataset file has to be in the local directory, ie: /home/user/

I would suggest to modify the dataset specification to specify that the Dataset file must be in the same directory than the .sigmf-meta file.

[777arc]

Good catch, it was always meant to refer to the same dir as meta, not some arbitrary local dir, I'll fix the wording

[777arc]

@lgoix note that that field is specific to NCD, which we tend to avoid using anyway

[lgoix]

I started to update the sigmf-python module to find the file, but I wanted the Spec to be updated before.