alternate relations then parentidentifier

some of the iso and geopython tooling allows to model alternate linkage between records then parentidentifier, this is of interest when the linkage has a different nature.

in iso, this type of linkage is managed via the [aggregation-info](http://www.datypic.com/sc/niem21/e-gmd_aggregationInfo-1.html) element. OWSlib is able to extract this info. pycsw captures this information in the record.relation field.

would be interesting to add this option to the pygeometa model, probably close to the parentidentifier element? aggregationinfo uses a [code list](https://standards.iso.org/iso/19139/resources/gmxCodelists.xml#DS_AssociationTypeCode) to identify what type of relation is involved, it could be interesting to add that option (but OWSlib and pycsw currently don't capture this information).