Get WHO small state membership status from ISO3 codes.
Source:R/member_states.R
is_who_member_small.Rdis_who_member_small() takes in a vector of ISO3 codes and returns a logical vector
on whether that country is a small WHO member state or not. In this instance,
small member states are defined as member states with a total population <=
90,000 in mid-year 2018, according to the World Population Prospects.