Enhancing XML- and JSON -conversion logic for metaschema constructs

[wendellpiez]

OSCAL has usnistgov/OSCAL#609 with model changes (renaming set-param to set-parameter). Integrating new testing has also revealed an error in data conversions, exposed in the sample SSP.

This PR introduces changes to the metaschema implementation to support this repair.

Committer Notes

A successful build will show data conversions from XML samples into JSON, and vice-versa, and round-tripped back with no data loss, as shown in the build scripting.

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