capture attributes info on iso19139

in #159 we've added the option to capture attribute info on mcf, a next step would be to capture this information also in the iso19139 documents generated from pygeometa. I'm starting this thread here to hear some views on this.

## current practices
- at [iso19115-3](https://www.iso.org/standard/32579.html) it is possible to embed featurecatalogue information
- at [iso19139:2007](https://www.iso.org/standard/32557.html) this was not yet fully possible, the [featuretypes element](http://www.datypic.com/sc/niem31/e-gmd_featureTypes-1.html) (which seems intended for this) is encoded as gco:GenericName_PropertyType, which many software can't handle well. Instead, people are generally referencing a dedicated [iso19110:2005](https://www.iso.org/standard/39965.html) record as [featurecatalogue-citation](http://www.datypic.com/sc/niem31/e-gmd_featureCatalogueCitation-1.html) holding the attributes information

## implementation options for iso19139:2007
- No action > endorse the iso19115-3
- have a separate output scheme to export the same mcf as iso19110:2005 and provide a mechanism to establish linkage between the 2 exported records
- develop and endorse an extension to iso19139:2007 which fixes the aspect of embedding a featurecatalogue by overriding the gco:GenericName_PropertyType of gmd:featureTypes with a proper iso19110:2005 featuretypes type

## schema information in other metadata conventions
- The data package convention contains a [schema property](https://specs.frictionlessdata.io/data-resource/#resource-schemas) which points to the relevant schema file
- The Stat DCAT AP profile adopted conventions to [embed attribute and dimension](https://github.com/SEMICeu/StatDCAT-AP/blob/59108dd298fa699cc270d8125242e34e1b15dc1f/Release%201.0.1/StatDCAT-AP_1.0.1.ttl#L283-L291) information, adopted from [SDMX](https://sdmx.org/)