add support for granular citation elements like author, year, journal etc.

[jhpoelen]

to help support datasets that contain citation elements in separate fields (e.g., author, year, journal), support bibtex / citation style sheet inspired data elements to parse citation elements.

For example see excerpt from https://github.com/JohnLlewelyn/random-forests-for-predicting-predator-prey-interactions-in-terrestrial-vertebrates below:

targetTaxonName	sourceTaxonName	authors_source	Year	Journal
Nymphicus hollandicus	Falco hypoleucos	Janse et al	2015	Corella
Rattus villosissimus	Elanus scriptus	3rd year Dickman Lab project reports: n = ~500	unpublished data
Melopsittacus undulatus	Elanus scriptus	3rd year Dickman Lab project reports: n = ~500	unpublished data