Now retrieve_sumario() provides informative error if a cve is provided
instead of a date (#17).
New functions to access the supplements, suplemento_cve() and sumario_suplementos().
These are only accessible for 3 months.
New function valid_code() returns a logical value instead of an error.
New function last_date_boe() returning the date of the last valid publication.
Better handling of dates (new fecha_anulacion).
retrieve_sumario() keeps all columns even if they are empty.
_cve. This affects anuncio, disposicion and specially sumario*.retrieve_document to retrieve and tidy any document from BOE.NEWS.md file to track changes to the package.