ProcessMetadata should have a single process identifier

[javierestevez]

Hello,

The class ProcessMetadata can hold a list of process identifiers but per the Peppol spec there should only be one. I tried to see where the factory methods that take a list of process identifiers are used but they seem not used other than within the class itself and using Collections#singletonList.

Can we safely assume that there will always only be one value? If so, is this something you can foresee to fix in a upcoming major version?

Thank you,
Javier