Implement XML element grouping on field or assembly sequences

[wendellpiez]

User Story:

Analogous to the way Metaschema offers features on the JSON side, it offers an attribute permitting the designer to indicate that a grouped sequence of fields or assemblies, when represented in XML, should be not an unwrapped sequence, but with an explicit wrapper.

Currently the attribute is named @xml-behavior and its permissible values in Metaschema are GROUPED and UNGROUPED, the latter being the default.

With GROUPED specified,

<model>
  <field name="citation" max-occurs="unbounded">
    <group-as name="citation-list" xml-behavior="GROUPED"/>
    <flag name="flag"/>
  </field>
</model>

instead of corresponding to

<citation flag="blah">one</citation>
<citation flag="blah">two</citation>

will be

<citation-list>
  <citation>one</citation>
  <citation>two</citation>
</citation-list>

Goals:

Implement this feature so that designers using metaschema can specify this representation.

Dependencies:

Note that in XSD, the declaration of such a field or assembly will correspond to two element declarations not only one (both citation and citation-list in this example).

Cardinalities should correspond. A minOccurs of 0 should result in an optional wrapper with a required child while when minOccurs is 1 or more, both should be required.

Also see #467 for renaming xml-behavior since strict XML processors might not approve of that name.

Finally, unit testing these features will need to be integrated into CI-CD (on two levels, validating schema production and validation schema functionality on instances), which will be very useful going forward.

Acceptance Criteria

• Unit tests have been developed and deployed to demonstrate the specified behavior
  • This includes both unit tests for XSD production and tests of the XSD functionality in instances (unit testing for validity or not)
• Metaschema XSD generation supports the functionality and unit tests pass (or fail) correctly
• A Pull Request (PR) is submitted that fully addresses the goals of this User Story. This issue is referenced in the PR.
• Integrate these unit tests into CI-CD.
• The CI-CD build process runs without any reported errors on the PR. This can be confirmed by reviewing that all checks have passed in the PR.

[wendellpiez]

We must also make name change(s) related to this since the name xml-behavior is not XML-compliant (since it starts with 'xml'). Possibilities: as-json and as-xml, in-json and in-xml ...

<group-as name="citation-list" as-xml="GROUPED" as-json="SINGLETON_OR_ARRAY"/>

[wendellpiez]

This is implemented in PR #484.

We need unit tests.

[david-waltermire]

This was completed in PR #492.