Repairs and new features for schema and converter generators

[wendellpiez]

Schemas and converter generators have been refurbished and tested with more extreme cases of valid Metaschema imports.

This PR will address at least usnistgov/OSCAL#829 usnistgov/OSCAL#792 #66 #49

Includes:

• Better represention of EMPTY elements (as assemblies not a datatype)
• Support for ANY in the XSD production and JSON Schema production ("additionalProperties"=true)
• Aliasing - different names for the same type
• Homonymy - different types with the same name

Committer Notes

Integration testing over a much wider range of samples is working.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you squashed any non-relevant commits and commit messages? [instructions]
• Do all automated CI/CD checks pass?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them? All changes are in keeping with stipulated Metaschema semantics
• Have you written new tests for your core changes, as applicable?
• Have you included examples of how to use your new feature(s)?
• Have you updated all website](https://pages.nist.gov/metaschema) and readme documentation affected by the changes you made? Changes to the website can be made in the website/content directory of your branch.

[david-waltermire]

LGTM!