DCAT-US writer: support describedByType field

[jwaspin]

Support DCAT-US describedByType field.

Conditionality: conditional (upon presence of describedBy)
Namespace: pod: "https://project-open-data.cio.gov/v1.1/schema#" ??

Description

The machine-readable file format (IANA Media Type also known as MIME Type) of the dataset’s Data Dictionary (describedBy).

Usage notes

This is used to identify the media type (IANA Media Type also known as MIME Type) of the URL used for the dataset’s describedBy field. This should be specified if describedBy is not an HTML webpage.

mdJSON mapping

For simplicity, leave blank implying html page, community decision needed whether to support other format types using mime type and in the form of "application/pdf"