Review SubstanceType and CAS numbers

[montanajava]

A specific substance can have multiple CAS numbers. We are currently putting these under alternateId. This results in multiple alternateIds (which is allowed by the XSD), with the same sourceSystemId (which is also allowed by the XSD).

Is this what we want? Do we want a special PSM substanceType which explicitly denotes CAS numbers. They main reason for asking is because if we have duplicate sourceSystemIds in an Identifier, then it really cannot be an Identifier at all. This may be okay, and may be just a philosophical instance of something not being entirely clean.

[montanajava]

ProductCatalog is already using this. We can regard that as a strong statement that this is a solid requirement and verified at least in that context.