Fix id_short checks and default values
id_short constraint checks are broken on multiple branches. Additionally, they seem not to be standard compliant w.r.t. optional/mandatory-ness. And, the default value for optional id_short attributes in the constructor arguments (on V3.0RC02) should not be "NotSet", but None.
For mandatory id_short (according to the standard) we could still allow
id_short == None as long as the object is not part of a Namespace (i.e. as long as
parent is None).