Re-think outputs of ft_get wrt each source

[sckott]

Right now ft_get() gives back slots for a plos, entrez, bmc, elife, pensoft, arxiv, and biorxiv. However, many sources can be gotten via entrez and bmc (well, bmc journals that is). Should the output have a named slot for the actual journal of the result, or if getting something via Entrez, just have it in the entrez slot in the output?