iso19139 aggregate-dataset-* implementation is missing

[ccancellieri]

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?

[ccancellieri]

Here is an example of a metadata aggregated to another by UUID, this implies that both metadata are on the same catalog.

<gmd:aggregationInfo>
<gmd:MD_AggregateInformation>
<gmd:aggregateDataSetIdentifier>
<gmd:MD_Identifier>
<gmd:code>
<gco:CharacterString>194------------------8029-4583--------------------086</gco:CharacterString>
</gmd:code>
</gmd:MD_Identifier>
</gmd:aggregateDataSetIdentifier>
<gmd:associationType>
<gmd:DS_AssociationTypeCode codeList="http://standards.iso.org/iso/19139/resources/gmxCodelists.xml#DS_AssociationTypeCode" codeListValue="crossReference"/>
</gmd:associationType>
</gmd:MD_AggregateInformation>
</gmd:aggregationInfo>