harvesting-kit: Adds nations field while harvesting

* Adds add_nations_field to utils.py to check affiliations and add
  the corresponding nation. 'HUMAN CHECK' if something went wrong

* Adds '--extract-nations' flag to valid arguments, since currently
  only SCOAP3 is going to use the nations field

Signed-off-by: Martin Vesper <martin.vesper@cern.ch>