make the form of vodml-ids mandatory

[pahjbo]

There was a long discussion in this thread http://mail.ivoa.net/pipermail/dm/2023-March/006364.html on this topic and it would seem that effectively making Appendix C normative would enforce what the standard VO-DML tools have been doing and remove any doubt that results from the description of VODML-IDs in the main body of the standard.

There is a secondary issue of whether the VODML-IDs are necessary at all given that they are created from their position in the VODML document hierarchy.

[pahjbo]

the schematron rules have been updated to check that appendix C is followed in the model.