Support Dataset Series serialization

[amercader]

See #298 for context

CKAN dataset series support is provided by the new ckanext-dataset-series extension.
At the ckanext-dcat level, we want to expose these entities as dcat:DatasetSeries rather than dcat:Dataset (for DCAT v3 profiles or higher). When receiving a data_dict, the serialize functions can check the type of the dataset to check if it's a Dataset Series and the in_series field to check if it's a dataset that belongs to a series, as well as the series_navigation information if present.

Series should include the dcat:first and dcat:last properties, and datasets dcat:previous and dcat:next (all of them if present).

Dataset Series were introduced in DCAT v3 so these changes should be implemented in EuropeanDCATAP3Profile._graph_from_dataset_v3() (DCAT AP v3 properties also applied to higher versions).