XML/JSON converter generator adjustments

[wendellpiez]

These address #174 and #175.

Changes:

• JSON-to-XML converter now supports (optional) $json stylesheet (runtime) parameter, so the JSON can be passed in directly as a literal data stream. $file is still available (designating a file path). If both are provided, $json is used in preference, unless it is an empty string, in which case $file is examined.
• Also repaired a nuisance bug reported by user @vikas-agarwal76 a few weeks ago (thanks!) - in addition to confusing docs (we repaired in Improvements to converter readme docs OSCAL#1027), a spurious warning message was produced due to a preventable template clash.

Committer Notes

Several improvements were made to the runtime interfaces for both converters (JSON/XML), without changing syntax -- better error messages, no spurious errors under SaxonC etc. (Thanks also to @danielnaab for contribution.) Operationally, nothing should change.

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?
• 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]

Thanks!