ICAT schema: add attributes for persistent identifiers

[RKrahl]

As discussed in the F2F meeting in November. I suggest that every item that may need to be unambiguously referenced from outside of ICAT should have a persistent identifier. At least, there should be a field in the schema that allows to store such a PID.

From the classes in the schema, I would identify Investigation, Dataset, Datafile, DataCollection, User, Instrument, ParameterType, and Sample as candidates that may need to be referenced. Investigation, Dataset, Datafile, DataCollection, and User already have suitable attributes to store a PID, doi and orcidId respectively. That leaves Instrument, ParameterType, and Sample. So, I suggest to add an attribute pid of type String [255] to these last three classes.