Add NIST SP 800-53 in YAML format

[redhatrises]

NIST SP 800-53 in yaml format which is part of #68 and #203

[iMichaela]

This is the first draft of SP 800-53 rev4 catalog represented in YAML. The conversion was dome from JSON. The team is encouraged to provide additional feedback.

[wendellpiez]

I like what I am seeing here. There are still a couple of important issues to be sorted through, mainly related to prose and arbitrarily-ordered content (including but not limited to mixed inline content).

How to approach this: I could work with @redhatrises trading examples and test cases, to explore possibilities. Or we could go straight to a larger group -- because similar issues are salient on the JSON side (where we are casting to markdown to address this) and also because a robust XML/YAML pathway (including tooling @david-waltermire-nist is contemplating) will depend on these decisions.

The acid test is a robust, lossless two-way conversion between XML OSCAL and YAML OSCAL and back again, analogous to what we are doing with the JSON (with the help of the metaschema).

cc @iMichaela @anweiss and @akarmel

[wendellpiez]

@redhatrises also I'd love to see the conversion script. Python?

A detail to add is that we have been aided in XML-JSON conversion by XPath 3.1 functions that execute the syntactic mapping. Without anything similar on the YAML side, we need to develop this capability -- and this script appears to be a good start (even if it reads the JSON not the XML).