Add dcat:CatalogRecord as optional

[leightonlc]

I would appreciate it you would consider adding the dcat:CatalogRecord class https://www.w3.org/TR/vocab-dcat/#class-catalog-record to the next version of POD (DCAT-US). The National Transportation Library has its own catalog, where we create catalog records for publications and datasets. Each of these records has a unique accession number (record identifier) that uniquely identifies an item from others of similar name, such as a publication that is published annually, or a dataset that has nearly the same name as the publication to which it is related. I would like to be able tie the dataset metadata record back to its catalog record. This is a feature we make use of when we mint DOIs for our datasets. However, the current POD V1.1 schema does not allow me to record that catalog accession number. It seems to me that the dcat:CatalogRecord class does give me an imperfect way to record that information. If you could take a look at that please, I would appreciate it. Thank you!