Enhancing XML- and JSON -conversion logic for metaschema constructs (u…

…snistgov#36)

* Work in progress on metaschema-related updates towards OSCAL SSP conversion roundtrips.
* added required top-level property to all unit-test JSON schema files
* Added XSD targets for unit testing
* oXygen project file for metaschema testing
* Updated project file
* Test coverage and planning docs
* added unit test for collapse
* fixed a bug in the test script causing expected schema files to not be compared.
* Addressing Issue usnistgov#11: JSON Schema missing constraint at root
* Adding metaschema documentation. usnistgov#16 usnistgov/OSCAL#596
* Added CI/CD files for website.
Added Github issue templates.
* Addressing usnistgov#12, supporting collapsibility of fields.
* Fixed website build error.
* Updating website contents and CI/CD configuration.
* Patch to catch up metaschema implementation to latest changes in the OSCAL repo.
* fixed broken links in documentation
* updated repository documentation to fix broken links
* added configuration support to build scripts
* Implemented content converter generation in build scripts
* Removed references to local files for usnistgov/OSCAL#566
* integrated schema generation unit tests with new build scripting
* Now accounting for json-value-key/json-key combination

Co-authored-by: David Waltermire <david.waltermire@nist.gov>