need doctype backtracing/linking feature as part of doctype filter
When a user specifies a return type for document types, only those document
types specified are returned. There should be an option to trace the links
between query hits (on filtered doc types) and the requested document types.
For example, if a query hits an eml-variable metadata document, but the user has
only requested eml-dataset type documents be returned, the query engine should
trace the "relations" element, or its successor, to determine if there is an
eml-dataset document associated with the hit. If so, the catalog should return