Security markings

[CDietleinNTIA]

Suggest adding the ability to more granularly control the level of security/sensitivity of information in metadata, and the ability to differentiate the security level of (for example) an IQ .sigmf-data file vs. its corresponding .sigmf-meta.

Potential ideas:

• Separate namespace to make machine reading simpler
• classification becomes meta_classification (highest in the file) and only exceptions to it (lower classifications) must be marked, although anything can be marked
• Hierarchical classification, e.g., meta_classification applies to everything and is overruled by classification at the object level
• New data_classification (required) to specify classification of .sigmf-data file
• Add a paragraph at the bottom of the main SigMF-NS-NTIA page that mentions this, once implemented
• Add a simple example in ntia-core section 4