New Methods
* Get metadata of all plays as CSV with `Corpus.metadata_csv()`
* Get cast of a Play as CSV with `Play.cast_csv()`
* Get relations of a Play as CSV, gexf, graphml with:
* `Play.relations_csv()`
* `Play.relations_gexf()`
* `Play.relations_graphml()`
* Get list of plays having a character identified by Wikidata ID with `DraCor.plays_by_character_wikidata_id(wikidata_id)`
* Get a mapping of play Ids and normalized years for a Corpus with `Corpus.normalized_years()`
Bug fixes
* Change data type from written, printed and published years to strings to cover entries such as "1842/43".
* Fix `Corpus.summary()`
* Fix `DraCor.sparql()`