Based on a predetermined set of global points, this function finds the country of
occurrence.
This can be useful for checking the validity of a point by comparing the returned country
to the country listed with the occurrence record.
If a point falls close to the boundary between two countries, the names of the nearby
countries are returned.
This function will not be of much value if the point falls in the ocean, as it will
return the country that is closest, regardless of how far away it is.