iso19139 aggregate-dataset-* implementation is missing

Hi,
I'm seeing that that we are correctly extracting the following informations from the iso19139:
# 'aggregate-dataset-identifier'
# 'aggregate-dataset-name'
# 'aggregate-dataset-type'

But then the base implementation of the get_package is not providing a corresponding map as package.

I was going to implement these as resources but I think it would be formally better IF we can start using relations between packages.

The fields above are used normally to link existing metadata to the metada shipping these informations, so we may use **aggregate-dataset-identifier** to reference other metadata by **GUID**

Thoughts?