ISO code regularization - may want to have a dedicated function

[bpbond]

See discussion in #99 , in which @pkyle notes this will be tricky. For later follow-up.

[ssmithClimate]

Just a note to talk to the CEDS team. We have an expanded list of countries and also more complete population data that might be helpful. (and also by then improved energy downscaling from IEA regions)

[rplzzz]

I think that we definitely want to have a dedicated function for this, precisely because it is tricky. We should do it right, once and for all, instead of expecting developers to do it themselves (and probably getting it wrong because it's so hard to do right).

[bpbond]

When we come back to this - see discussion and solution (use of change_iso_code) done in #331 .