Add short-form human or machine readable field to annotations

[jacobagilbert]

As part of my review on #120, i noted that the label field is really a generally useful feature that should be in core. We currently have comment, however this is not specified as short form and may very well be a short sentence or something otherwise not machine readable.

We have the rfml.sigmf-ext but that has a number of problems for general use (mostly that if you use that extension, you are required to include an array of labels that fit into a single, fixed (for a given dataset) label hierarchy for every single annotation to remain compliant.

I propose the addition of an optional core:label or core:description field to annotations for a short-form human or machine readable string. The latter is consistent with the field currently in global and avoids conflicting with the label field in rfml.sigmf-ext. I also propose that there be an optional core:included_labels field in global for users to specify what labels are in the file (software utilities can easily do this automatically) to avoid requiring users to iterate over all annotations to know what is in the file.

[Teque5]

@jacobagilbert I think core:label implies a short description better than core:description. Maybe we want to limit this field to a certain number of characters for use in guis (20?).

If it were me I might put an ITU designator in there, which is only 8 chars: 11K2 F3E.