Bugfix/article ID parsing issue

[iacopy]

Thanks for the useful and well written library!

This minimal fix make the parser returning only the paper pubmed id.
Before, also the ids of the citations papers were returned.(they are within the ReferenceList element of the xml).

Fixes #22

An alternative XPath to be used:
path = ".//PubmedData/ArticleIdList/ArticleId[@idtype='pubmed']"

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