DvQuantified - check complete interface and implementation

[Tim Cook]

Reference openEHR Datatypes Information Model Specifications Release 2.1.1

[Sergio Miranda Freire (sergio-lampada)]

The method valid_magnitude_status is not correctly implemented:
     it should return true if the argument value is in ('<', '>', '=', '<=', '>=', '~'), false otherwise
Then this method should be used to enforce the invariang Magnitude_status_valid:
 magnitude_status /= Void implies valid_magnitude_status(magnitude_status)

magnitudeStatus is not correctly implemented:
It is a String object and should assume one of the values in ('<', '>', '=', '<=', '>=', '~') if present.

The method magnitudeExists is not necessary.

accuracyUnknown is a method, not an attribute.