Basic OSCAL Metaschema load test

[nikitawootten-nist]

Committer Notes

Fixes #27

• [x] Fix field, flag, and assembly exportation

All Submissions:

• [x] Have you selected the correct base branch per Contributing guidance?
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Changes to Core Features:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] Have you written new tests for your core changes, as applicable?
• [x] Have you included examples of how to use your new feature(s)?

[nikitawootten-nist]

Note: while on a call with @david-waltermire-nist we encountered problems accessing the loaded document's exported fields, flags, and assemblies. This issue will now include fixing this functionality in its scope.

[nikitawootten-nist]

Reviewed as a team on 9/9