Aggregation concordance building functionality with
> - method get_correspondance_dict in CountryConverter
> - function agg_conc which build concordance matrices in various
> formats
> - notebook tutorial
Include Eora(26) country codes
CountryConverter accepts a parameter only_UNmember to restrict the
concordances to UN member countries.
Fix and close Issue 28 (Wrong ISO3 code for Palestine) and 25
(Non-standard codes).