Backward compatible fixes

[jbrown-xentity]

This is to evaluate and provide context and testing for GSA/data.gov#5543.

This PR does a number of things.

• Provide more detail into documents failing validation
• Updates JSON schema definition in various places to make more backward compatible and cleaning bad references, including:
  • [fix]: Force keyword list to be an array of strings
  • [fix]: Update identifier to be a string and to be required, not array or null.
  • [backward compatible]: Allowing contactPoint to be an array or a simple object
  • [backward compatible]: Force keywords to not be blank, and the keyword array to not be empty
  • [backward compatible]: Allow modified field to contain duration value compliant with ISO time intervals definition (ie annually)
  • [backward compatible]: Allow spatial to be either an array of locations or a direct reference to a location
  • [backward compatible]: remove catalog requirement for description, publisher, and title (as this information isn't really used)
  • [backward compatible]: remove organization (dataset publisher field) requirement for prefLabel, as in practice this seems duplicate of name, which is already required.

This can be evaluated by running python validate_jsonschema.py from the dcat-us3 folder. Changing things to bad values, or removing required fields, should show the failing job with errors.