Make it easier to refer to controlled vocabularies for MeasuredVariable

[RKrahl]

Discussed in today's meeting: MeasuredVariable is just free text. Obviously it is difficult to constrain it to a controlled vocabulary, since there none that would applicable to all domains. But we could add subproperties like measuredVariableName and measuredVariableIdentifier similar to InstrumentType in order to make it easier to point to domain specific controlled vocabs.

Furthermore, we could collect somewhere (in the WhitePaper) lists of recommended vocabs to use.

[BaptisteCecconi]

We should mimic what is proposed for the subjects in Datacite, see #67 (comment), allowing then to cite any ontology. One could also think of using the I-ADOPT (or equivalent) framework, but I'd rather push for including I-ADOPT (or equivalent) metadata in MeasuredVariable ontologies. This way we keep the PIDINST schema simple.

[RKrahl]

Just for the reference, what we have in PIDINST 1.0 now is a simple (multi valued) property without subproperties:

ID	Property	Obligation	Occ	Definition	Allowed values, constraints, remarks
10	MeasuredVariable	R	0-n	The variable(s) that this instrument measures or observes	Free text

One option would be to mimic what we already do for other properties allowing to link with PIDs, e.g. Owner, Manufacturer, Model, InstrumentType: change that into a complex property having subproperties in the following way:

ID	Property	Obligation	Occ	Definition	Allowed values, constraints, remarks
10	MeasuredVariable	R	0-n	The variable(s) that this instrument measures or observes
10.1	measuredVariableName	R	1	Full name of the variable	Free text
10.2	measuredVariableIdentifier	O	0-1	Identifier used to identify the variable	Free text, should be a globally unique identifier
10.2.1	measuredVariableIdentifierType	O	1	Type of the identifier	Free text

We could discuss if we want to add additional subproperties, such as scheme and schemeURI (valueURI would be the same as measuredVariableIdentifier).