Extend prepopulate plugin to support copying of full container attributes from a source specification to a target. Unlike existing prepopulate rules this will require setting of a "source" specification rather than a display template. It will be assumed as a default that both source and target containers have the same structure and sub-element names (ie. it's the same container on a different record). This will allow replication of container values between parent and child records.
Provision should also be made for simple mapping of elements in one container structure to another, allowing copies between different containers and/or omission of specific sub-elements.