Add society sets

[xrotwang]

In addition to datasets and phylogenies, a third kind of contribution should be added: society sets; the prime example being the SCCS, cotributed by Murdoch. Thus, CONAN should become a separate dataset, with 75 variables coded for the SCCS society set.

@kirbykat We'd need a list of society sets, and citations for them, or do they map to the current datasets EA, Binford, SCCS and WNAI? We also need to split up the existing datasets (again mainly SCCS, I think) into citable subsets a la CONAN.

[xrotwang]

The implementation is there, but the data is not yet divided appropriately.

[xrotwang]

Addressed in the new scheme of things via DatasetWithSocieties and DatasetWithoutSocieties in pydplace.