Datatype mismatch between Schematron and SVRL in @role and @flag

[dmj]

In SVRL @‍role and @‍flag are defined to be of type xsd:NMTOKEN. In Schematron the corresponding attributes are defined as string. The datatype string is more permissive than xsd:NMTOKEN, leading to a situation where a value for e.g. @‍role is valid in Schematron but cannot be used in SVRL.

Proposed solution: Relax the datatype in SVRL to be string as well.