IOP parser: Change the field's source of published_date

[ErnestaP]

Now, the date_published is taken from: pub-date pub-type="ppub"><day>XX</day><month>YY</month><year>ZZZZ</year></pub-date> and it should be taken from pub-date pub-type="open-access"><day>XX</day><month>YY</month><year>ZZZZ</year></pub-date>