support for UN Comtrade country codes

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 only allowed the UN Comtrade country codes to be `origin`, not `destination`. Also, UN Comtrade actually also provides data for when countries started/ceased to exist (with their respective identifiers), I think this could be an interesting addition here as well for the future.

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).