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.