Nations: Affiliation adjustments

* check_nations.py now allows multiple affiliations. Those are
  added as individual subfields (100__w or 700__w).

* CERN is now added to NATIONS_DEFAULT_MAP. Picking CERN over
  Switzerland in case both are in the same affiliation.

* nations.py is now also uses the fields 100__w or 700__w to
  find the records affiliated to CERN.

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