Update JSON-XML conversion script production to current Metaschema model

[wendellpiez]

User Story:

As a developer I need a robust means to produce data converters for Metaschema-defined OSCAL, from XML to JSON and back. (YAML can potentially be supported as an extension on the JSON side.)

As a user I need to be able to perform such conversions and trust they work reliably.

Goals:

Having a new-improved XML-to-JSON pathway (see #51), we also need to go the other way, converting OSCAL JSON into the equivalent XML.

This capability was first prototyped for Milestone 1. However now we have a superior approach, much easier to maintain and test, as well as supporting new Metaschema features such as local definitions.

The new architecture performs conversion by transforming either XML or JSON into a common supermodel representation, which can then be (relatively trivially) serialized back into either syntax.

Dependencies:

For #51 we have built out most of the infrastructure for this including the supermodel serializers. Indeed the JSON -> XML conversion for a given Metaschema might conceivably be derived from the XML->JSON converter, only with match patterns and select expressions mapped from the XML variant into the JSON variant paths.

However, to port this logic over we also need to be able to convert paths addressing the XML into the Metaschema-equivalent paths for the JSON. Development of this capability is being tracked in usnistgov/metaschema-xslt#13.

Acceptance Criteria

• All website and readme documentation affected by the changes in this issue have been updated. Changes to the website can be made in the docs/content directory of your branch.
• A Pull Request (PR) is submitted that fully addresses the goals of this User Story. This issue is referenced in the PR.
• The CI-CD build process runs without any reported errors on the PR. This can be confirmed by reviewing that all checks have passed in the PR.

{The items above are general acceptance criteria for all User Stories. Please describe anything else that must be completed for this issue to be considered resolved.}

[wendellpiez]

Templates are being produced to produce "abstracted supermodel" representation from JSON source. They do not yet capture all the data correctly, for example in the case of fields whose values are given directly (because having no without flags), and perhaps in other positions. But close. Still working.

Test data running through a round-trip conversion - XML to JSON back to XML - serves for unit testing.

See usnistgov/OSCAL#633.

[david-waltermire]

This should be updated based on the changes summarized in #195.

[wendellpiez]

Work has been done, currently stable.