Add SigMF file format support

[zuckschwerdt]

Some very basic support for SigMF file reading and writing.

Supports -r, -w, and -S, e.g. -w myfile.sigmf or -r myfile.sigmf or -S sigmf:unknown.

[merbanan]

Looks ok but it is too much for an in-depth review.

[zuckschwerdt]

Thanks, yes, needs some seroius refactoring to integrate smoother. Very much work-in-progress.

[adulau]

Thanks a lot for your work! We will also do some tests.