Support profiling of union properties

[jechterhoff]

A <<union>> is used to model a choice between a set of options. These options are typically modelled as properties of the union, with multiplicity 1..x.

The PMT should allow profiling these options (even if an option has multiplicity 1..x), much like it is done for codes of a code list.

A consistency check would then be to ensure that a union that belongs to a profile has at least one option that also belongs to the profile.

[azahnen]

Profiling of mandatory properties is allowed for codelists, enumerations and unions.
The consistency check to ensure that at least one of the mandatory belongs to the profile when the class does only applies to enumerations and unions.