Adding transformation for including DCAT-AP themes about iso-19139-to-dcat-ap

[andrea-perego]

@AntoRot , I've added a tentative implementation of the mappings in PR #7

To be reviewed before merging.

[AntoRot]

@andrea-perego I performed some tests and the implementation seems work.

The transformation could be also extended to cover the INSPIRE themes encoded with gco:CharacterString element at least in English language, leaving to the reusers to further include additional extensions to take into account the themes expressed in other MS languages.

[andrea-perego]

Thanks, @AntoRot .

About extending the mapping as you suggest, the version of the GeoDCAT-AP XSLT used by the European Data Portal (EDP) is already doing it, for all languages. So, we could build upon the approach used there.

The question is whether this feature should be part of the reference GeoDCAT-AP XSLT, or it is rather more in scope with customised versions of the XSLT used in production environments (as the EDP one).

For instance, applying this extension will significantly increase the complexity of the XSLT with a possible impact on its efficiency. By contrast, the URI-based mapping has not these issue, and it is designed for metadata records following the INSPIRE recommendations of using URIs for the themes.

What is your view on this, @AntoRot ?