Add invariants to DV_COUNT and DV_QUANTITY

The RM spec doesn't tend to include invariants for the existence of primitive type fields that are non-reference types, i.e. Boolean, Integer, Real, Character etc, because in programming languages, these fields can't be Void/ null, they always have a value. String fields and all other types pretty much are reference fields.

It's only in XML and serialised formats that these non-reference fields could possibly be absent.

Currently we don't have a clear policy on how to indicate that within the RM, or even if we should.

I think this PR should be related to the XSD ITS.