Add identity constraints for xs:IDREF fields

[jmwaura]

The current ID references are unconstrained, potentially allowing improperly referenced entities to pass validation.
As an example, this is valid under the current schema:

<vip_object>
  <precinct id="precinctA14">
    ...
  </precinct>
  <polling_location id="pollinglocationA14">
    ...
  </polling_location>
  <street_segment id="streetsegmentA14.538">
    <precinct_id>pollinglocationA14</precinct_id>
    ...
  </street_segment>
</vip_object>

Ideally, the schema should use identity constraints (or some similar mechanism, if one exists) to ensure that the above example would fail validation.

[JDziurlaj]

We in the NIST CDF WG looked into this a while back, and ran into an issue with IDREFS. We have decided to go down the path of using Schematron instead. You can take a look at what a ruleset looks like here as well as an explainer.