Add DOI field check for article.doi

[eseiver]

Right now, self.doi in the Article class (article_class.py) relies on the user entering the DOI correctly & the accompanying local filename matching. It would be good to also check that the DOI in local XML DOI field matches. That field is located in an element at the xpath location /article/front/article-meta/article-id with element.attrib = {"pub-id-type": "doi"} in element.text.