Factor out the HRM

[palemieux]

With the publication of IMSC HRM REC, the HRM clause can be replaced with a link to the [IMSC HRM REC].

[nigelmegitt]

I think our discussions before were along the lines of removing the normative HRM conformance requirement altogether, alongside the HRM algorithm specifications, and informatively pointing readers to the existence of IMSC-HRM, for use if appropriate to their needs.