Add dcat:theme and dcatap:hvdCategory on Dataset series

[matthiaspalmer]

Some dataportals will choose to hide datasets that are part of dataset series in the main search interface, or at least provide a filtering option to do so. The most obvious reason for doing this would be to provide a better search experience and avoid datasets that are percieved as duplicates drowning out other search results.

A consequence of this is that the dataset series will sometimes be displayed alongside datasets in a search result. This means that they should be smilar enough to be searched for and filtered just as datasets. The most obvious aspect of filtering is publishers and categories and categories are currently missing.

Hence, I think it makes sense to add both dcat:theme and dcatap:hvdCategory on dataset series.