Update DCAT-US namespaces

[hmaier-fws]

The current dcat-us writer assigns the "dcat" namespace to all exported elements. "DCAT" is used by the DCAT-US schema and others to represent elements described by the W3C Data Catalog Vocabulary.

The DCAT-US schema (https://project-open-data.cio.gov/v1.1/schema/catalog.jsonld) uses several namespaces. The namespace used for DCAT-US specific elements is "POD" (project open data). Perhaps we should use the namespace for the authoritative source of the field. For example, the language field (language: "dc:language") is actually sourced from Dublin Core terms namespace (http://purl.org/dc/terms/).

This may simplify future implementation of a generic JSON-LD export.

Example

• The current DCAT-US writer accessLevel field is assigned a "dcat" namespace.
• accessLevel is not an element described by the W3C DCAT vocabulary
• the DCAT-US JSON-LD schema uses the "pod" namespace: accessLevel: "pod:accessLevel"