Include physical object identifier such as inventory number

[huberrob]

To allow to link between the physical object (instrument) and its digital representation, it would be good to have a property such as 'physicalObjectIdentifier' for Ids such as 'inventory number' etc.. The schema now has relatedIdentifier as well as alternateIdentifier. It is not clear which one should be used in case the 'real' physical object should be identified.