Update countrycode_data.csv

I added the numeric country codes for UN Comtrade data (Source: http://unstats.un.org/unsd/tradekb/Knowledgebase/Comtrade-Country-Code-and-Name). Most are identical to the UN country codes, however some differ. As some countries have ceased to exist or merged with others, I would suggest to allow the UN Comtrade country codes only to be origin, not destination. Also, UN Comtrade actually also provides data for when countries started/ceased to exist (with their respective identifiers), I see you've started working on something along these lines with the states2011.csv.

Additionally, I suggest a minor correction: the former German Democratic Republic became part of the Federal Republic of Germany, which retained the iso3c and iso2c codes, however changed the iso3n code to 276 (before 278 (GDR) and 280 (FRG), respectively). Also, it should be "Wuerttemberg" not "-burg".