Metaschema composition improvements, with unit testing

[wendellpiez]

Committer Notes

Addresses #76 and #81.

The Metaschema Schematron has also been updated so that local definitions are found for purposes of name cross-checking.

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? See Support tagging top-level metaschema definitions as 'local' to exclude them when imported #76 and Metaschema composition unit testing #81
• Have you written new tests for your core changes, as applicable? This is mainly tests
• 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.