Add Subject

[RKrahl]

Add Subject as an optional complex property having the subproperties subjectName, subjectIdentifier and subjectIdentifierType. Close #72.

This proposal mimics what we already do for other properties allowing to link with PIDs, e.g. Owner, Manufacturer, Model, InstrumentType: a complex property having subproperties in the following way:

ID	Property	Obligation	Occ	Definition	Allowed values, constraints, remarks
XX	Subject	O	0-n	Subject or keyword describing the instrument
XX.1	subjectName	O	1	Subject name or text	Free text
XX.2	subjectIdentifier	O	0-1	Identifier in the case an external vocabulary	Free text, should be a globally unique identifier
XX.2.1	subjectIdentifierType	O	1	Type of the identifier	Free text

Note that the new property is added with the Id XX for the moment. This needs to be replaced with the actual Id that the property will get at the point of merging this pull request.

[RKrahl]

Discussed in today's meeting:

• there were discussion about the name of the property and whether its purpose is well defined.
• we didn't have enough time left in the meeting to settle this discussion.
• postpone the final decision to the June monthly meeting to allow for some more time to discuss here.

[RKrahl]

Please vote!

We need your feedback in order to decide whether to add this property. Please vote by clicking on the emoji at the bottom of this comment and select either:

👍 -- accept: add this property
👎 -- reject: do not add it
👀 -- abstain

Please add your vote by end of Juin. We will discuss the outcome of the voting in the July monthly meeting.

[rebeccaringuette]

Also generally in favor of this. Should we support the identifier for the vocabulary as in #90? e.g., SchemeURI in datacite?