Add sources for SCCS data

[xrotwang]

SCCS data uses the same sources per society as EA, so the sources field in data.csv should be filled in accordingly from an appropriate variable in EA, e.g. Subsistence economy: hunting.

[kirbykat]

Yes, for now sources are general across the majority of the variables in EA, and all variables in SCCS. I'm hoping an RA can eventually tackle links between specific variables and references.