Replace reference fields in datafiles with citekeys

[SimonGreenhill]

I realised that we have the reference in index.csv. Do we need it there? it's a pain to curate, and we should just be able to list the bibtex cite key from sources.bib

[xrotwang]

I agree. In the worst case, e.g. if we wanted a particular linearization of the bibliographic data as citation, we could stick this as additional field into the bibtex file as well. And we should do this for both, datasets and phylogenies.

[SimonGreenhill]

Just realised that this also goes for variables.csv which includes sources (much easier and smaller to have a citekey there), and I'm sure there are other files with references in them too.

[xrotwang]

For the SCCS variables we actually have both, bibtex file and mapping between text citation and bibtex key. Not for the other datasets, though.

[kirbykat]

@SimonGreenhill In the case of SCCS, the references in APA format are
just placeholders for better variable definitions (so, when you select
a variable like "Fish", it says "Further information: Murdock and
Morrow, 1970, etc etc". The Murdock and Morrow article defines the
variables and codes.