Add support for EuropePMC

[hbhargava7]

Inspired by discussion here: https://twitter.com/christlet/status/1615093358883655680?s=20

europePMC mirrors Pubmed but also includes preprints. May be a more useful base DB. Implementation would require creation of equivalent functions such as author_query, author_query_from_row, etc. with europePMC search syntax. Should also look into whether europePMC API would be a better avenue for creating feeds.