identifiers for Requirements per OGC ModSpec

[rob-metalinkage]

Sections such as 5.5.2.2 have a list of requirements with a local informal list numbering - these are virtually impossible to cite reliably when implementing solutions.

It would be preferable to follow the OGC ModSpec which requires URIs for requirements classes and individual requirements throughout the document.

This could possibly be done as a final editorial step against a final draft. Or introduced in a PR in order to facilitate change tracking later.

If requirements are based on section number they may be hard to maintain.