Test for app wit reference xml:ids and hash references

[ebeshero]

@eraisanen and I figured out that we're not seeing errors for missing hashmarks or possibly broken/erroneous @wit in the play files. See if we need a Schematron constraint for this in our ODD schema.

[ebeshero]

I think we've fixed this. I tested it on @eraisanen 's Foscari file and if I try to break the @wit attribute value (enter a wrong value) the schema properly informs me with this error message:
"Every reading witness (@wit) after the hashtag must match an xml:id defined in the list of witnesses in this file!"
@eraisanen If you are still seeing the error, let me know and show me which file / where you're seeing it. Thanks!